Generate Release Notes in GitLab Releases

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### Problem to solve

Gitlab users want to easily find/read release notes relevant to them, such as tier, the stages they use or specific items of interest (upgrades, deprecations and removals) in release announcements. Gitlab team members (such as Marketing and Sales) want to successfully find/share relevant release notes with Gitlab users and prospects. 

The following pages deliver Gitlab's release notes organized in various ways, forcing internal and external Gitlab users to reference various pages to collect the information they seek:

- **Release Post Marketing Blog"**: https://about.gitlab.com/releases/2020/06/22/gitlab-13-1-released/
- **Gitlab Releases"** https://about.gitlab.com/releases/ archived ==> https://web.archive.org/web/20200714130736/https://about.gitlab.com/releases/
- **GitLab upcoming releases"** https://about.gitlab.com/upcoming-releases/
- **Upcoming features for paid tiers"** https://about.gitlab.com/direction/paid_tiers/
- **X-Y Preview"** https://about.gitlab.com/releases/gitlab-com/
- **Gitlab releases"** https://gitlab.com/gitlab-org/gitlab/-/releases/v12.10.0-ee

### Intended users

Current users, prospects, internal Gitlab team members

### Proposal

Improve and leverage Gitlab's in product Release feature https://gitlab.com/gitlab-org/gitlab/-/releases to serve as the "go to" SSOT for both SaaS and Self-hosted release notes, such that it can essentially replace the need for the various pages listed above.

**Benefits**

- Deliver a better user release notes experience internally/externally
- Boost process efficiency by decoupling the requirements of "release notes" from a "marketing blog post"
- Enable Product team to focus on feature/supporting content success 
- Enable Marketing to focus on how, when and where else to best expose the release notes as part
- Enable Sales to clearly communicate via one thorough/sortable SSOT for existing and potential customers

**Requirements:**

To successfully replace the various release pages listed above, the Release feature https://gitlab.com/gitlab-org/gitlab/-/releases needs to:

|   | Requirement                                                                                                                                                                            |                                                                                                                                                                                                                        |
|---|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 1 | Display or provide quick access to content blocks that currently only surface in the [Release Post Marketing Blog](https://about.gitlab.com/releases/2020/06/22/gitlab-13-1-released/) | - primary features<br>- secondary features (inclusive of recurring release notes such as Omnibus, Chart and Runner improvements)<br>- bugs<br>- performance improvements<br>- upgrades<br>- removals<br>- deprecations <br> - Issue to follow: https://gitlab.com/gitlab-org/gitlab/-/issues/234009 |
| 2 | Individual items should link through to:                                                                                                                                               | - changelogs<br>- documentation<br>- epics <br>- issues     <br> Issue to follow: https://gitlab.com/gitlab-org/gitlab/-/issues/217145                                                                                                                                                            |
| 3 | The page should be sortable / filterable by:                                                                                                                                           | - version<br>- plan type<br>- stage<br>- category   <br> Issue to follow: https://gitlab.com/gitlab-org/gitlab/-/issues/234007                                                                                                                                                                   | 
| 4 | Be continually up to date with features as they become available for SaaS (similar to or link to... https://about.gitlab.com/releases/gitlab-com/)                                     |  Epic to follow: https://gitlab.com/groups/gitlab-org/-/epics/4160                                                                                                                                                                                                                      |
| 5 | Include a way to highlight "Upcoming" features for Self-hosted, which currently happens on https://about.gitlab.com/releases/gitlab-com/ but is hard to discern                        | Epic to follow: https://gitlab.com/groups/gitlab-org/-/epics/4160                                                                                                                                                                                                                       |
| 6 | Include a way to search release notes                      |   <br> - Issue to follow: https://gitlab.com/gitlab-org/gitlab/-/issues/227178                                                                                                                                                                                                                     |
| 7 | Include a way to still be able to pull content blocks and generate the Marketing release post blog in it's current format                      |   <br> - Issue to follow: https://gitlab.com/gitlab-org/gitlab/-/issues/234009                                                                                                                                                                                                                     |

Story points: 8.0