Update equivalencies.rst

Removed (nearly) repeated text.  I kept the most recent additions.
Corrections to ensure Distance can be compared.  Closes #2206
fixed AiryDisk2D model
updated changelog
updated changelog
Updated AiryDisk2D model docs
defined module-level constant for AiryDisk2D model; fixed bug with peak-pixel amplitude
changed AiryDisk2DKernel input parameter from width to radius to match AiryDisk2D model
Updated test for AiryDisk2D model for non-unity amplitude
Added reference link for AiryDisk2D model
defined AiryDisk2D constant in the model class
Fix for Astropy Issue 2240
Removed check for negative RA.
Also test d**2 -> Quantity; better change-log entry -> 0.3.2
Merge pull request #2250 from mhvk/distance-comparison

Corrections to ensure Distance can be compared.  Closes #2206
Merge pull request #2242 from pllim/conesearch-ra-check-fix

Fix for Cone Search erroneously raising an error for valid RA
Merge pull request #2093 from larrybradley/airy

fixed AiryDisk2D model
Merge pull request #2248 from larrybradley/patch-3

Update equivalencies.rst
Expanded VO catalog and database functionalities. Moved VO exceptions to a separate modules in client and validator. Changed inheritance to super. Updated doc and tests. Fixed codes for Python 3 compatibility and to complete coverage. Applied review comments.