[WIP] Add documentation

[colinepiloquet]

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)
• [ ] 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?

documentation tool update

What is the current behavior?

Technical documentation is hosted on the website powsybl.org and in the powsybl.github.io repository.

What is the new behavior (if this is a feature change)? This PR creates the readthedocs website and adds a few pages to show what can be done.

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

• [ ] Yes
• [x] No