Add documentation reference file entry in kmp.inf/kmp.json

[mcdurdin]

In order to remove our dependence on the precise filename welcome.htm, it would be sensible to allow for inclusion of an entry in kmp.inf/kmp.json that references the documentation file, just as we have for ReadmeFile. Similarly for usage.htm. If the entry is missing then fall back to the default name.

This change would impact all platforms.

[mcdurdin]

See also:

• feat(developer): Support blank or missing .model_info/.keyboard_info 🎺 #9351, somewhat related

[mcdurdin]

This has been implemented in the file format and compiler, in #9709. Don't think it is used in any of the apps yet.