Clone IRDB into RTD build, update RTD versions

[teutoburg]

As is done in ScopeSim, avoids downloading the IRDB during the docs execution.

Spectra are still downloaded, this should be fixed in SpeXtra (and ScopeSim_Data, see AstarVienna/ScopeSim_Data#32).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (8981e19) to head (3be4e7e).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #111   +/-   ##
=======================================
  Coverage   98.78%   98.78%           
=======================================
  Files           6        6           
  Lines         493      493           
=======================================
  Hits          487      487           
  Misses          6        6           

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