Refactor and enhance documentation throughout the codebase

[johnh2o2]

Update and improve all documentation for cuvarbase, including docstrings, the README, and usage examples. Ensure all public functions and classes have clear docstrings, and that the README accurately reflects current capabilities and installation instructions. Add or update example notebooks or scripts for users.

[johnh2o2]

v1.0 status. Delivered under this umbrella in the packed release:

• PDM: full Sphinx page (docs/source/pdm.rst) + API docs (community PR PDM refactorization and kernel speedup #62)
• Sphinx autodoc now covers every post-0.2.6 module (bls_frequencies, cufinufft_backend, the experimental TLS modules, the memory/base subpackages)
• Power-spectrum convention sections in bls.rst/lomb.rst (Document + maybe add some flexibility for powerspectrum convention #17)
• README significance pass + benchmark docs (docs/BENCHMARK_RESULTS.md), with claims either traced to repo data or rephrased
• Docstring corrections wherever the audit found claims contradicting behavior (TLS limits, cuFINUFFT backend, batch BLS, pinned-memory naming, FAP heuristics)

Deferred (v1.1): the all-public-API docstring audit and updated example notebooks. Leaving open for those.