Addons: add documentation for `CustomScript`

We need to document this addon and put some examples on how to solve common issues with this feature.

* Refs: https://github.com/readthedocs/readthedocs.org/pull/11758
* Requires: https://github.com/readthedocs/readthedocs.org/issues/11774
* Requires: https://github.com/readthedocs/ext-theme/issues/527
* Requires: https://github.com/readthedocs/addons/pull/474