DOC: document adding datasets to a PSF library

[sapunyangkut]

Summary

• document how to append an aligned dataset to an existing PSF library
• explain optional spectral collapse and saving the updated correlation matrix
• note that persisted images and filenames must remain in sync with the matrix

Closes #7.

Validation

• parsed pyklip/rdi.py with the standard-library AST and verified the documented
  method signature and implementation behavior
• asserted the new RST heading, method reference, code block, options, and
  persistence guidance against the source
• git diff --check

The local Python environment does not include the project's scientific runtime
dependencies or Sphinx, so tests/test_rdi.py and a full documentation build
could not be run locally.

AI assistance

OpenAI Codex was used to investigate the issue, prepare the documentation, and
run local validation. The final wording, source alignment, and diff were
independently reviewed before submission.