Fix links redirecting to powsybl website documentation

[colinepiloquet]

Please check if the PR fulfills these requirements

• [x] The commit message follows our guidelines

Does this PR already have an issue describing the problem?

No

What kind of change does this PR introduce?

Docs update

What is the current behavior?

Some links refer to powysbl.org documentation that has been deleted.

What is the new behavior (if this is a feature change)? Replaced by readthedocs website. However, I didn't manage to use Intersphinx...

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

• [ ] Yes
• [X] No