Transformer documentation updates

[bbjay]

Updated the documentation to reflect that BackgroundTransformer is the new default transformer.

• added information about BackgroundTransformer
• removed the outdated & deprecated code example
• removed outdated info about methods (only applies to transformRequest, as explained in next paragraph)
• updated TOC

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

[ueman]

Thanks!