Document writing third party packages

Resolve #2043

Add the following sections under the "Third Party Resources" topic:
- “About Third Party Packages”
- “How to create a Third Party Package”
- “Existing Third Party Packages”

Also, refactor all links to maintain consistency with the rest of the docs.
Reminder to self to tweak this slightly. This is less about difficulty in removing stuff vs adding stuff, and more about distributing maintainership to the authors.