This introduces new directives for RBTools documentation that deals with

multi-commands:

rbt-subcommand
rbt-subcommand-usage
rbt-subcommand-options

This works just like the normal command equivalents, but are used to

document subcommands within a command.
Where possible, most of the code is shared between the command and

subcommand versions.
Upcoming changes will begin making use of these new directives.

Built new docs using these commands. Verified that the results were

correct, sections were in the right place (for both commands and

subcommands), and that references were all correct.