feat: support output groups

[kaycebasques]

Fixes #19

[kaycebasques]

@tpudlik FYI re: https://pwrev.dev/263952

[kaycebasques]

Notes from @tpudlik:

• First thought was to use output attributes because it would remove the need for intermediary targets and allow us to use something like :doxygen_build.html instead.
• But it turns out that output attributes don't support directories so output groups are the only viable solution for now.
• The output groups should be documented in the rule string. (And probably elsewhere in the rules_doxygen docs.)

[TendTo]

I have to admit I had not used OutputGroups before. It does seem like a welcome addition for whoever may need it, thanks.

[TendTo]

If you feel like there is nothing more to add I will merge the pr

[kaycebasques]

Thank you! Nothing more to add on my end so you're welcome to merge whenever