Script that checks documentation

We need a script that checks if documentation has all needed parts up to date:
- [ ] if all documented functions are really in code
- [ ] if all interactive functions are really documented and indexed
- [ ] if keybindings are matching their functions
- [ ] if all keybindings are documented and indexed

Two way checking. Note that to make it checkable in texi there is probably a need for some nice macros.

No files, tests, or entry points are named; start by locating the documentation and keybinding sources and clarifying the intended script and texi macros. Done means bidirectional checks cover documented functions, interactive functions, keybindings, and their indexing, with mismatches reported.

Written by the indexing model from the issue text.