Closes #7451

I considered including freshrss:load-more (#1278) in the document, but held off after struggling with how to write it since it is dispatched to a different location.

How to test the feature manually:

1. I confirmed that I can trigger the add_load_more_listener of [ReadingTime] Add options: Reading speed, Source metrics Extensions#300 .

Pull request checklist:

• clear commit messages
• code manually tested
• unit tests written (optional if too hard)
• documentation updated

Additional information can be found in the documentation.