Update documentation to include yarn in our dependencies

What does this MR do?

Adds Yarn dependency requirement to our install/upgrade from source documentation.

Also adds Yarn rake tasks and dependency checks to things like gitlab:assets:compile so they will fail with less cryptic error messages when yarn is unavailable or our dependencies are out of date.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Previously if someone tried to run rake gitlab:assets:compile without first updating the installed node_modules, webpack would fail citing a missing module but a user may not know to use yarn install to fix this. Now it will first run yarn check to ensure required dependencies are there before attempting to run webpack.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• [ ] API support added
• Tests
  • [ ] Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Related: #27613 (closed)