This PR addresses https://github.com/a11yproject/a11yproject.com/issues/1425 and partially addresses https://github.com/a11yproject/a11yproject.com/issues/1360. It:
- Removes the Contribute page and promotes the Spotlight to the primary site navigation.
- Adds Collections to the project navigation.
Additionally, this PR:
- Adds dates to the announcement posts' frontmatter.
- Re-orderers the announcements order so that the most recent announcement is listed first.
- Removes the Changelog from the Announcements page