Add note about docs

What does this MR do?

While looking for the docs, I realised we removed them from the repo in favour of having them in https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/development/pages/index.md. However, I only saw that because I could see the commit history.

This MR adds a note and links to the new current location.

It also removes the dependency_decisions.yml file which hasn't been maintained in a long time.

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines