Docs improvements following M2 release.

This PR contains cherry-picked improvements from the M2 release branch.

Included:
* M2 release notes.
* Additions to download page, including links to release notes & getting started, and more obvious getting started path.
* Re-inclusion and relocation of eclipse.include.md.
* Creation of the release notes directory.
* Addition of the div.warning css class used in the website hotfix for M1 MySQL known issue.

Not included:
* **aledsage**: 047fb719a0d2371628bc4ceeaad0ead2da83fa9c *Update catalog.xml to point at maven-central*, as builds from master will be SNAPSHOTs held on sonatype.org, not mavel-central.