mkdocs: initial html documentation

[m4ra]

Tasks

• [ ] PR name contains story or task reference
• [x] Documentation (docs and inline)
• [ ] Tests (including n+1 and django_assert_num_queries where applicable)
• [x] Changelog

[m4ra]

@goapunk we need to get the mkdocs.yml in the main branch and then I can generate the dedicated github branch for deploying the documentation website on github pages

[m4ra]

@goapunk I addressed your comments, added info from the README in the index page, and changed the theme.