Fix version switch URL (#4629)

[AkshitBoora]

Fixes the incorrect versions_json_url in Sphinx configuration.

Updated the URL to use the correct docs.mdanalysis.org endpoint.

Closes #4629

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📚 Documentation preview 📚: https://mdanalysis--5251.org.readthedocs.build/en/5251/

[codecov]

Codecov Report

❌ Patch coverage is 94.13068% with 53 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.84%. Comparing base (900d20b) to head (092d0e9).

Files with missing lines	Patch %	Lines
package/MDAnalysis/coordinates/TRZ.py	89.69%	9 Missing and 1 partial ⚠️
package/MDAnalysis/visualization/streamlines_3D.py	64.70%	6 Missing ⚠️
package/MDAnalysis/analysis/align.py	50.00%	4 Missing ⚠️
package/MDAnalysis/coordinates/base.py	93.44%	4 Missing ⚠️
package/MDAnalysis/lib/transformations.py	42.85%	4 Missing ⚠️
...ckage/MDAnalysis/analysis/encore/confdistmatrix.py	62.50%	2 Missing and 1 partial ⚠️
package/MDAnalysis/analysis/encore/utils.py	33.33%	1 Missing and 1 partial ⚠️
package/MDAnalysis/topology/tpr/utils.py	92.59%	1 Missing and 1 partial ⚠️
package/MDAnalysis/analysis/bat.py	85.71%	1 Missing ⚠️
...e/MDAnalysis/analysis/encore/clustering/cluster.py	75.00%	0 Missing and 1 partial ⚠️
... and 16 more

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #5251   +/-   ##
========================================
  Coverage    93.83%   93.84%           
========================================
  Files          180      180           
  Lines        22473    22473           
  Branches      3189     3189           
========================================
+ Hits         21088    21089    +1     
- Misses         923      927    +4     
+ Partials       462      457    -5     

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