Add documentation. Add entry point installation.

[rfrenchseti]

• Fixes #11
• Fixes #13

Support PyPI-based installation by a user:

• Add "Publish to PyPI" (triggered by a GitHub release) and "Publish to Test PyPI" (triggered manually) GitHub actions.
  • Neither of these can be tested until after this pull is merged so further changes may be required.
  • After this pull is merged they will each need to be triggered and the appropriate PyPI administrative work done, including adding the appropriate SECRET tokens to this repo. (@rfrenchseti will do this)
• Add main entry point for pds4_create_xml_index.py in pyproject.toml.

To test installation, open a terminal that is unrelated to this project and:

cd <some temporary directory>
python3 -m venv venv
source venv/bin/activate
pip install <dir>/rms-pds4indextools

At this point you should be able to type:

pds4_create_xml_index --help

Add documentation:

• Modify README.md to follow our standard format and remove most information on how to run pds4_create_xml_index.
• Add a docs directory supporting Sphinx.
• Update requirements.txt to support Sphinx.
• Update Sphinx documentation to include the original contents of README.md, now converted to RST format, and updated to show current command line options and also fleshed out with more details and examples.
• Further review and work on the documentation is required, and several areas are marked with FIXIT tags.
• To build the documentation (required after every change to a .rst file):

cd docs
make html

• To view the built documentation, use your file browser to go to <dir>/rms-pds4indextools/docs/_build/html and double-click on the index.html file. Then view the docs in your browser window.
• For information on formatting documentation in RST format, see https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html
• As soon as this pull is merged, I will install the necessary hooks for it to generate web pages for readthedocs automatically.

Miscellaneous

• Minor changes to argparse help messages.
• Added new class MultilineFormatter which allows hard newlines to be placed in help strings for better formatting.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 40.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 72.36%. Comparing base (fc88c40) to head (4c8a0a1). Report is 1 commits behind head on main.

Files	Patch %	Lines
pds4indextools/pds4_create_xml_index.py	40.00%	8 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #25 +/- ## ========================================== - Coverage 73.22% 72.36% -0.87% ========================================== Files 1 1 Lines 564 579 +15 Branches 128 130 +2 ========================================== + Hits 413 419 +6 - Misses 130 138 +8 - Partials 21 22 +1 ```

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