Fix dead link to iTools documentation

[alicecaron]

Please check if the PR fulfills these requirements

• 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)
• A PR or issue has been opened in all impacted repositories (if any)

What kind of change does this PR introduce?
Documentation fix

What is the current behavior?
The link to iTools read he docs is dead from the main page here. Dead link: https://powsybl.readthedocs.io/en/latest/itools/index.html

What is the new behavior (if this is a feature change)?
he link for the iTools documentation is correct: https://powsybl.readthedocs.io/projects/powsybl-core/en/stable/user/itools/index.html

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

• Yes
• 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: