doc: introduce rocks.troubleshooting + UX tweaks

[teto]

Looking at #424, I think we can better guide the user in case of error, instead of displaying Could not find diffview.nvim. Search for 'dev' version? y/n: , we could show a vim.ui.select:

1. Install the "dev" version (when we know it exists)
2. Install via rocks-git (and install rocks-git if not available)
   < mention rocks.troubleshootingi or an url towards how to add plugins to luarocks / link to luarocks packaging requests #111 >

[github-actions]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

Proper conventional commit scoping:

• For example, fix(installer): some installer bugfix

• Pull request title has the appropriate conventional commit prefix.

If applicable:

• Tested
  • Tests have been added.
  • Tested manually (steps in PR description).
• Updated documentation.

[mrcjkb]

Install via rocks-git (and install rocks-git if not available)

I think it's fine just to mention the possibility of using rocks-git in the docs.
rocks.nvim shouldn't be "aware" of any external modules and there's no way to infer the repo owner.