Export plugin filters

[kecnry]

Description

This pull request fixes the availability of plugin products being available to export from the export plugin if not added to the viewer by removing the default layer_in_viewer filter on the dataset component. This also:

• excludes non-plugin data via a filter (to be debated if this is better or worse than the messages)
• excludes spectral subsets via a filter
• adds language to the sections of the export plugin which includes explanations of what is supported, hopefully to avoid confusion

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Attention: Patch coverage is 90.00000% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 88.81%. Comparing base (f1ba4a6) to head (0148b76).
Report is 16 commits behind head on main.

Files	Patch %	Lines
jdaviz/core/template_mixin.py	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2780      +/-   ##
==========================================
+ Coverage   88.72%   88.81%   +0.09%     
==========================================
  Files         110      110              
  Lines       16353    16433      +80     
==========================================
+ Hits        14509    14595      +86     
+ Misses       1844     1838       -6     

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[rosteen]

Looks like you missed the Tables section, I think that should also have a caption like this for consistency. Other than that, this PR looks good.

[rosteen]

Thanks!

[haticekaratay]

Thanks for your work! Tested across different viz and works great!