Add documentation around custom utterances

[Dref360]

Description:

Added a page around custom utterances, we support perturbation and saliency.

Found the following issues:

• Fix some name collision around the name of the future
• get_saliency wasn't getting pipeline_index
• No need to check if the task is done, we never save the result.

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge
it.

• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• FRONTEND TYPES. Regenerate the front-ent types if you played with types and routes.
  Run cd webapp && yarn types while the back-end is running.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[gabegma]

Awesome, thank you!!