New ReadTheDoc documentation

Please check if the PR fulfills these requirements

[X] The commit message follows our guidelines
[ ] Tests for the changes have been added (for bug fixes / features)
[X] Docs have been added / updated (for bug fixes / features)

Does this PR already have an issue describing the problem? No

What kind of change does this PR introduce? New technical documentation

What is the current behavior? The technical documentation was available on https://www.powsybl.org/index.html and hosted on https://github.com/powsybl/powsybl.github.io.

What is the new behavior (if this is a feature change)? The technical documentation is now hosted in the present repository and will be available on powsybl.readthedocs.org

Does this PR introduce a breaking change or deprecate an API?

[ ] Yes
[X] No

If yes, please check if the following requirements are fulfilled

[ ] The Breaking Change or Deprecated label has been added
[ ] The migration steps are described in the following section

What changes might users need to make in their application due to this PR? (migration steps)

Other information:

powsybl / powsybl-afs

New ReadTheDoc documentation #150

Quality Gate passed