Merge: doc/commands: introduce `main` related commands

This PR introduce new doc commands related to executable files:
* `CmdMains`: list all executable files from a package
* `CmdMainCompile`: return the `nitc` command to compile an executable file
* `CmdTesting`: return the `nitunit` command to test a package
* `CmdManSynopsis`: return the command synopsis from its manpage
* `CmdManOptions`: return the command options descriptions from its manpage

Pull-Request: #2656
Reviewed-by: Jean Privat <jean@pryen.org>