Migration from ReadTheDocs to Mkdocs

[muscariello]

We have a ready version of the doc using mkdocs instead of readthedocs.
To maintainers, please do not merge any new PR until #106 is not merged
@aduminuc @jnapper7 @jubarbot-cisco please do not merge any new modification as we're changing documentation engine.

[aduminuc]

@muscariello , we have to update a link in the AGNTCY Agent Directory to adapt to this migration. Let's coordinate on Monday to make sure that the releases of docs and Agent Directory are done at the same time.