docs: add notes about external packages

[paul-soporan]

What's the problem this PR addresses?

• @yarnpkg/sdks and @yarnpkg/pnpify didn't have READMEs.
• The command details of our external packages didn't have a for more details footer linking back to the main page in the docs.
• The pages of commands added by plugins didn't link to the README of the plugin.
• The pages of commands added by external packages didn't mention the fact that they need to be installed separately.

Fixes #3809.

How did you fix it?

• Added READMEs.
• Added one.
• Linked the name of the plugin to its README.
• Added a section about it, linked the external package name to its README, and mentioned how it can be installed.

Checklist

• I have read the Contributing Guide.

• I have set the packages that need to be released for my changes to be effective.

• I will check that all automated PR checks pass before the PR gets reviewed.