Move deprecations build process and templates to gitlab-docs

The current process for adding a deprecation looks like this:

1. Add your item as a YML file in the main gitlab project
2. Run a rake task (bin/rake gitlab:docs:check_deprecations) to rebuild the Deprecations page
3. Commit and merge both the YML and the generated page

Full version: https://about.gitlab.com/handbook/marketing/blog/release-posts/#update-the-deprecations-and-removals-docs

This makes changes to the templates difficult to test since they're in a different project than the rest of the Docs site, and it's an unusual workflow for writers since other pages don't require that additional step of doing a build process over in the gitlab project. People frequently have problems running the rake commands in their local environments.

Proposed new version:

• Deprecation authors continue adding deprecations as YML in the gitlab project
• We recreate the templates in gitlab-docs
• We remove the rake task and templates from gitlab -- authors will no longer need to be responsible for this step

Considerations

• Release post automation: A script for creating the release post relies on specific markup in the page template. We'll need to make sure to test and adjust that when making any changes to the template. See gitlab-com/www-gitlab-com!124921 (merged).
• If we build this page from gitlab-docs, it won't be available in /help. Is that acceptable?