Meta: Update documentation

This is a task to start thinking about the documentation updates that need to be made. Individual tasks should be created for the different sections of this and referenced in the issue summary.

Some immediate goals:

• #1402154: Update main handbook page
• Split handbook page into several pages by desired outcome or feature
• #1505130: Document solutions to common Fill PDF implementation problems
• Update fillpdf_merge_pdf() parameter documentation to match module.
• Record screencast for overriding the default rule.

Help is most welcome with these tasks. Anyone can edit handbook pages.