Move common code from the template's `docs/_scripts/macros.py` to a new `frequenz.repo.config.mkdocs.macros` module

[llucax]

What's needed?

We need to remove boilerplate code from the user's repository, otherwise when the user customize these files and there is a new version, upgrading is much harder as the changes need to be merged.

Proposed solution

Move the boilerplate code to library code:

• Create frequenz.repo.config.mkdocs.macros.
• Move the functions _CODE_ANNOTATION_MARKER, _slugify and _hook_macros_plugin to the new module and make them public.
• Update the templates to remove these and use them from the library, and also remove the mypy ignore for Markdown, as the user code doesn't need to import it directly anymore.

Use cases

No response

Alternatives and workarounds

No response

Additional context

No response