Warn about `src/*.md` files that are not in `SUMMARY.md`

[szabgab]

Problem

If the user creates an md-file in the src folder but forgets to add it to the SUMMARY.md it is totally disregarded. IMHO it would be better if the build warned about it.

In addition there could be a configuration flag e.g. unused_files that would have values: silent, warn, error
to control this behavior the default being warn.

Related to #3047

[ehuss]

Thanks for the report! I think this is essentially a duplicate of #2358, so closing in favor of that. Adding a warning here may be difficult due to things like include files, or other extensions that may be reading extra files.