Add discoverability metadata

[Zethson]

Makes pertpy easier to find for search engines, LLM agents and citation tooling.

PyPI — adds keywords and Issues / Changelog / Forum / Publication project URLs.

CITATION.cff — enables GitHub's "Cite this repository" button and lets citation tooling resolve the Nature Methods article. Validated against schema 1.2.0 with cffconvert.

Docs

• sphinxext-opengraph was already a doc dependency with ogp_site_url and ogp_image configured, but was never added to extensions. The live docs therefore serve no OpenGraph tags and no meta description at all.
• html_baseurl is now taken from READTHEDOCS_CANONICAL_URL. Because the build uses custom build.jobs commands, Read the Docs does not inject a canonical URL itself, so /en/latest/, /en/stable/ and all nine tagged versions currently compete as duplicate content.
• docs/llms.txt is served by Read the Docs from the root of the domain and gives agents a curated map of the API and tutorials. Every symbol it names was checked against pertpy.tools.__all__.
• The index page heading now matches the README, so the <title> is no longer pertpy — pertpy.

hatch run docs:build (which passes -W) builds clean, and the output contains canonical links, OpenGraph tags and meta descriptions on every page.

Repository topics were separately expanded from 3 to 13 in the repository settings; that is not part of this diff.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.72%. Comparing base (40dfeb6) to head (ae663c4).
⚠️ Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1067      +/-   ##
==========================================
+ Coverage   74.15%   78.72%   +4.56%     
==========================================
  Files          52       52              
  Lines        7291     7290       -1     
==========================================
+ Hits         5407     5739     +332     
+ Misses       1884     1551     -333     

see 7 files with indirect coverage changes

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