Add "What's new" menu item in top navigation
### Problem to solve
Currently, we rarely highlight new features within GitLab. There is no inline link to our monthly release post that matches the version of the GitLab instance.
##### Thoughts
- Promote our release post or something similar cc: @tauriedavis
- Make sure people actually update the instance whenever a new release is out
- Better place to show version info than on the help page
- Notify of new version
### Proposal
* For the first iteration, we add a "What's new?" item to the Help dropdown as depicted in the design below.
* Tell instances about new things that have been added in their current version by adding static release notes that live locally on the instance.
* For a first iteration, we can just generate these from `CHANGELOG.md`.
* Tell instances about new things that have been added in future versions:
* I think that the boring/most straightforward approach here is to just always link to the latest major release post. For a first iteration, we can link to the latest major release by linking to a static URL on `about.gitlab.com` that redirects to the most recent release post.
Future iterations:
* In the future, we can make this approach fancier and more friendly to read. Maybe by writing specific release notes from the release post.
* Create an endpoint on `about.gitlab.com` which receives a url param of the instance version. This would let us create a cool diff between your instance version and latest.
<img src="https://gitlab.com/gitlab-org/gitlab-foss/uploads/3e85d2398f270965e6e16f5a5d75ac36/variations.png" width=345px>
### Previous Proposal
* For the first iteration, we add a "What's new?" item to the Help dropdown as depicted in the design below.
* This new item will be placed between `Help` and `Contribute to GitLab`.
* The "What's new?" row should link to the release post for the installed version.
* For patch releases, the row will link to the blog post for the **corresponding** major or minor release (e.g. an `11.6.3` installation links to `11.6`)
* For release candidates, the row will link to the blog post for the **previous** major or minor release (e.g. a `11.6-RC3` installation links to `11.5`)
### Links / references
* https://gitlab.com/gitlab-org/gitlab-ce/issues/24479
* https://gitlab.com/gitlab-org/gitlab-ce/issues/23598
* https://gitlab.com/gitlab-org/gitlab-ce/issues/2359814381
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