Restructure ESA docs

[bsipocz]

This should fix the module based testing (the test infra requires the same subdirectory in the docs as the namespace to pick up modules for testing)

[codecov]

Codecov Report

Merging #2369 (a7b6124) into main (115b0aa) will not change coverage.
The diff coverage is 56.20%.

@@           Coverage Diff           @@
##             main    #2369   +/-   ##
=======================================
  Coverage   63.17%   63.17%           
=======================================
  Files         132      132           
  Lines       17189    17189           
=======================================
  Hits        10860    10860           
  Misses       6329     6329           

Impacted Files	Coverage Δ
astroquery/esa/iso/core.py	64.58% <ø> (ø)
astroquery/exceptions.py	0.00% <0.00%> (ø)
astroquery/gaia/core.py	71.94% <ø> (ø)
astroquery/image_cutouts/first/core.py	92.68% <ø> (ø)
astroquery/magpis/core.py	93.61% <ø> (ø)
astroquery/mast/services.py	79.56% <ø> (ø)
astroquery/atomic/core.py	35.77% <30.76%> (ø)
astroquery/utils/tap/taputils.py	67.85% <48.78%> (ø)
astroquery/alma/core.py	42.80% <55.55%> (ø)
astroquery/mast/missions.py	76.54% <75.00%> (ø)
... and 18 more

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