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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 ``` | [Flag](https://app.codecov.io/gh/spacetelescope/webbpsf/pull/917/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/spacetelescope/webbpsf/pull/917/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope) | `62.15% <ø> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=spacetelescope#carryforward-flags-in-the-pull-request-comment) to find out more.

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