Add docstrings for read_file and read_file_sc

[namurphy]

Replaces the one-line docstrings of read_file and read_file_sc in ccsds_reader_pipeline.py with full docstrings. These are the two public functions for reading L0 files into Python data structures.

Each docstring now has Parameters, Returns, Raises, and Notes sections:

• read_file reads SPC instrument packets (APIDs 0x351–0x354, 0x35E, and 0x35F).
• read_file_sc reads spacecraft housekeeping packets. Its notes describe how the spacecraft housekeeping definition file is chosen: first from the MET of the first packet, then from a <year>/<day of year> part of the path, and finally by falling back to the newest definition file.
• Both return a dict keyed by APID and then by mnemonic, raise RuntimeError when the file can't be opened or read, and write progress to standard output.

The documentation builds with --nitpicky --fail-on-warning.

🤖 Generated with Claude Code

[namurphy]

These docstrings were written by Claude. Nothing appeared to be glaringly wrong with them, but these will need careful review since I can neither confirm nor deny the veracity (truthiness?) of these docstrings. 👀

@joyvelasquez — could you read through these docstrings too, if you have the time? No worries if not.

Stuff that we aren't able to confirm should go as a comment into the code rather than be included in a docstring, perhaps. 🤔