DOC: Fix Imviz Contrast/Bias doc and other minor updates

[pllim]

Description

This pull request is to update some really outdated docs, particularly for Imviz, but not attempting to fix everything.

Rendered doc: https://jdaviz--1385.org.readthedocs.build/en/1385/imviz/displayimages.html#contrast-bias

Fixes #1360

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 change log needed? If yes, is it added to CHANGES.rst?
• Is a milestone set?
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[pllim]

Not sure why the dev job tanked but this PR does not touch any code, so it is unrelated.

[codecov]

Codecov Report

Merging #1385 (4d94d64) into main (8c3d3e7) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1385   +/-   ##
=======================================
  Coverage   84.91%   84.91%           
=======================================
  Files          91       91           
  Lines        8288     8288           
=======================================
  Hits         7038     7038           
  Misses       1250     1250           

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[kecnry]

Let's get this in, definitely an improvement over the outdated icons/text. Thanks!