feat: implement from_AlphaFold fetcher #3377

[jhawpetoss6-collab]

This PR implements the requested from_AlphaFold fetcher for MDAnalysis, allowing users to download and cache structures directly from the AlphaFold Protein Structure Database (#3377).

Changes:

• Added MDAnalysis.fetch.alphafold module with from_AlphaFold function.
• Integrated with pooch for robust caching and download management.
• Exposed fetch in the top-level MDAnalysis namespace for easier access.
• Supports PDB and CIF formats.

/claim #3377

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📚 Documentation preview 📚: https://mdanalysis--5335.org.readthedocs.build/en/5335/

[BradyAJohnston]

Closing due to AI policy.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.79%. Comparing base (9f6d169) to head (1f96c3f).

Files with missing lines	Patch %	Lines
package/MDAnalysis/fetch/alphafold.py	40.90%	13 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #5335      +/-   ##
===========================================
- Coverage    93.83%   93.79%   -0.05%     
===========================================
  Files          182      183       +1     
  Lines        22492    22516      +24     
  Branches      3199     3203       +4     
===========================================
+ Hits         21106    21118      +12     
- Misses         924      936      +12     
  Partials       462      462              

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