[WIP] Documentation on network modifications

[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

What is the current behavior?

There is no documentation about the network modifications in powsybl

What is the new behavior (if this is a feature change)? Starts some doc.

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

• [ ] Yes
• [X] No