Add documentation for resample_spec

[stscijgbot-jp]

Issue JP-3328 was created on JIRA by Melanie Clarke:

The ReadTheDocs pages for resampling contain minimal information about spectral resampling.

The ResampleSpecStep page, linked from the inheritance diagram on the main page, returns a 404 error. The resample_spec module also does not appear to be documented.

In particular, I was checking the external documentation for information about the output spectral WCS for NIRSpec data products, for work related to JP-3244.  I do not see the output transforms documented anywhere in the software documentation.

[stscijgbot-jp]

Comment by Melanie Clarke on JIRA:

Documentation updates for JP-3547 will fix the broken link for ResampleSpecStep and add the missing documentation for the ResampleSpecData class.

I have also added a note about the output frames available to the functions that produce the output WCS for NIRSpec and MIRI, and I have clarified which step parameters are used for spectral resampling and which are ignored.

Please let me know if there's anything else it would be useful to add.

[stscijgbot-jp]

Comment by Tyler Pauly on JIRA:

Fix included in #8596