Add the `mkdocs-include-markdown-plugin`

[llucax]

What's needed?

We need to be able to include parts of a markdown file in another.

Proposed solution

Add the mkdocs-include-markdown-plugin.

TODO

• Add the extension
• Reuse parts of the README in the docs #279

Use cases

Reuse parts of the README in the documentation, and avoid repeating / copy&pasting.

Alternatives and workarounds

No response

Additional context

PR adding this to Channels:

• Add some useful mkdocs extensions frequenz-channels-python#244