Compare userguide from branch (generation compatibility)

[Vrixyz]

This PR enables to easily see modifications made to user guides, without having to run diff scripts locally

• While this PR doesn´t change add much features compared to github's builtin diff, but will be useful for Migrate ncollide docs #2 which now generates the user guide.

This uses pull_request_target, which runs the CI on the base (target) repository, opposed to pull_request which runs on the fork repository. That's by github's design to limit security concerns, so we can use github token to comment.

⚠️ This is still an advanced technique which will require some care on future PRs, because we're running external code from the PR.

• For a minimal example, see add incorrect data to trigger diff Vrixyz/parry.rs#7 (comment).
• For real use-cases examples, see:
  • inject files + fix code snippets Vrixyz/parry.rs#1 (comment)
  • Migrate ncollide docs Vrixyz/parry.rs#6 (comment)

Note this new CI step doesn´t run for this PR, because:

• pull_request_target doesn't exist in the target branch, so it won't run.
• This could run if we used pull_request, but we wouldn't be able to comment h ere (a different repository from the PR branch.
• I didn´t modify anything in the watched path.

[Vrixyz]

add a description for this workflow