[SIGMON-543] use mkdocstrings instead of sphinx

Context

• Both mkdocs and sphinx were used to generate documentation:
• The goal is to use the package mkdocstrings and remove sphinx doc generation for the API.
• This will also unify the UI of the documentation instead of having one part of it using material theme and the other part having readTheDoc theme.

What was done

• Remove any .rst file
• Add API documentation using mkdocstrings

TODO

• This change is breaking one link in https://sigmon.web.cern.ch, I need to know how can this be addressed.

QA

• code was not unit tested, because it's impossible to test basically
• acceptance tests were written

Review objectives

• assessment of code functionality