Update docs to Documenter 1

[andreasnoack]

Summary

• Bump Documenter compat in docs/Project.toml from ~0.27 to 1.
• Add the single-argument fit(::Type{KaplanMeier}, ::Any) and fit(::Type{NelsonAalen}, ::Any) signatures to the API blocks so Documenter 1's stricter missing_docs check passes.

Test plan

• julia --project=docs docs/make.jl builds cleanly (only the expected "could not auto-detect the building environment" warning from deploydocs when run locally).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.32%. Comparing base (646828a) to head (773ab2a).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
- Coverage   99.32%   99.32%   -0.01%     
==========================================
  Files           6        6              
  Lines         297      296       -1     
==========================================
- Hits          295      294       -1     
  Misses          2        2              

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