docs(config options): example to limit registryAliases to one manager

[HonkingGoose]

Changes

• Show example of using a top-level manager object to limit the scope of a registryAliases block
• Add titles to the codeblocks to label each example properly

Context

Quote from #29747 (comment)

I think this issue still needs a fix because:

• Coming up with this solution is non-trivial
• The existence of top-level manager objects is not well-documented

I can't fix any code problems, that's for the maintainers. But I can add a example to the docs. 😉

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[renovate-release]

🎉 This issue has been resolved in version 37.431.5 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 37.431.5

Your semantic-release bot 📦🚀