elisp_manual() needs to accept additional inputs

[gutron]

As an example, magit/ghub's doc is an .org file which tries to pull in an org setup file: https://github.com/magit/ghub/blob/main/docs/ghub.org?plain=1#L12

There's no way right now to pass that setup file (.orgconfig) to elisp_manual to generate the texinfo.

[phst]

OK, I guess we want something like additional_inputs, similar to additional_compiler_inputs for cc_library etc.

[gutron]

The latest commit (9b31929) works and generates the texi docs correctly. Thanks!