Add IFU+datacubes docs page.

[mperrin]

• Add a new docs page for JWST IFU + data cubes simulations. This functionality was included in the 1.4 release, but we didn't really document it well anywhere yet.
• Also, fix minor nbsphinx formatting issue on the matching PSFs page, which was preventing the bulleted lists from rendering correctly in nbsphinx. A blank line is needed before the bulleted lists for nbsphinx to display them correctly.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 62.15%. Comparing base (d39fa3d) to head (490d6bf).
Report is 42 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #917      +/-   ##
===========================================
+ Coverage    61.85%   62.15%   +0.30%     
===========================================
  Files           15       15              
  Lines         6945     7016      +71     
===========================================
+ Hits          4296     4361      +65     
- Misses        2649     2655       +6     

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[obi-wan76]

Looks good, I ran the new notebook with no errors.