Cross-link new VS Code extension page in docs

The following discussion from !72322 (merged) should be addressed:

• @aqualls started a discussion:

  @phikai your concern is 100% valid. I like the idea of followup to garden in links to the extension from appropriate pages in the docs. I was hesitant to include them in this merge request because long-running docs MRs can become unpleasant to merge. I'll use this comment to spawn a new issue for just that.

Note to self: also add this new page to the left nav.