StaticFetcher() Implementation + from_pdb() refactor

[jauy123]

Fixes #5429 and #5431

Changes made in this Pull Request:

• Implements fetch.fetchers.StaticFetcher()
• Refactors fetch.pdb.from_pdb() to use StaticFetchers()

LLM / AI generated code disclosure

LLMs or other AI-powered tools (beyond simple IDE use cases) were used in this contribution: yes / no
From comment: #5436 (comment)

Per AI policy, I'm declaring that I used in AI in primary two ways: IDE autocomplete and help with reviewing the docstring used for the documentation to make it look more professional.

The one notable exception is that I used AI to help me draft test_multiple_downloads_existing_database and test_existing_database in testsuite/MDAnalysisTests/fetch/test_static_fetcher.py

I prompt is "Is there a unique way to determine the hash in order if two files are exactly the same including modification time in python using the standard library"

It gave code that used os.stat(), so I googled it and found pathlib.Path.stat().st_mtime in the official [docs](https://docs.python.org/3/library/pathlib.html#pathlib.Path.stat) which I then used manually for test_existing_database and test_multiple_downloads_existing_database with no AI.

PR Checklist

• Issue raised/referenced?
• Tests updated/added?
• Documentation updated/added?
• package/CHANGELOG file updated?
• Is your name in package/AUTHORS? (If it is not, add it!)
• LLM/AI disclosure was updated.

Developers Certificate of Origin

I certify that I can submit this code contribution as described in the Developer Certificate of Origin, under the MDAnalysis LICENSE.

[jauy123]

Docs need to be reupdated, but the code itself feel like it is in a completed state.

[read-the-docs-community]

Documentation build overview

📚 MDAnalysis | 🛠️ Build #33932891 | 📁 Comparing 1d1c8c4 against latest (83bc9a2)

  🔍 Preview build  

10 files changed · + 2 added · ± 8 modified

+ Added

• documentation_pages/fetchers/fetchers.html
• _modules/MDAnalysis/fetch/fetchers.html

± Modified

• genindex.html
• index.html
• py-modindex.html
• _modules/index.html
• documentation_pages/fetchers_modules.html
• documentation_pages/analysis/wbridge_analysis.html
• documentation_pages/fetchers/PDB.html
• _modules/MDAnalysis/fetch/pdb.html