Generate bindings map from code for the documentation

[nkh]

also check that all bindings are documented automatically

[nkh]

place markdown stubs in a directory , if no stub for it exists

stub: group + binding + bound function

group could be a sub directory to keep things tidy

mdbook support {{#include file.rs}}

[nkh]

Solution

• put a short help per command in a help module
• Setup verifies that all commands have help
  • warns if help is missing
  • warns if help for old commands is present
• help module is used by mdbook help application
  • generates snippets in markdown
    • deletes all snippets first
• markdown files include the snippets in the main documentation

[nkh]

rejected in a1edf0c