use canonical name of RTD config and enable nitpicky mode to catch broken links in docs

[zacharyburnett]

the ReadTheDocs documentation indicates that filenames other than .readthedocs.yaml are deprecated:

Some other variants like readthedocs.yaml, .readthedocs.yml, etc are deprecated.

[codecov]

Codecov Report

Base: 86.93% // Head: 86.93% // No change to project coverage 👍

Coverage data is based on head (2747487) compared to base (724ce0b).
Patch has no changes to coverable lines.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #430   +/-   ##
=======================================
  Coverage   86.93%   86.93%           
=======================================
  Files          25       25           
  Lines        3888     3888           
=======================================
  Hits         3380     3380           
  Misses        508      508           

Flag	Coverage Δ
unit	86.93% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gwcs/coordinate_frames.py	95.15% <ø> (ø)
gwcs/selector.py	92.94% <ø> (ø)
gwcs/spectroscopy.py	97.79% <ø> (ø)
gwcs/wcs.py	87.65% <ø> (ø)
gwcs/wcstools.py	80.30% <ø> (ø)

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