feat(extensions): add typedoc plugin markdown as peer deps of typedoc

[njfamirm]

What's the problem this PR addresses?

#6653

How did you fix it?

Read compatibility guide and comment in #6653
...

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.

[njfamirm]

The master branch builds failed ! so I can't fix bugs 🐛

[clemyan]

This does not seem correct. A core package should not depend on plugins unless the plugin is "blessed". It doesn't seem like that is the case for typedoc-plugin-markdown upon a cursory glance.

[njfamirm]

This does not seem correct. A core package should not depend on plugins unless the plugin is "blessed". It doesn't seem like that is the case for typedoc-plugin-markdown upon a cursory glance.

I just realized!
You're right.