Document the index convention of read_epw

[cdeil]

• Addresses read_epw data shifted 1 hour back #1156 (documents the behaviour, does not change it)
• I am familiar with the contributing guidelines
• I attest that all AI-generated material has been vetted for accuracy and is in compliance with the pvlib license
• Adds description and name entries in the appropriate "what's new" file in docs/sphinx/source/whatsnew for all changes.
• Pull request is nearly complete and ready for detailed review.

read_epw labels each EPW record with the start of its hour (EPW hour 1 becomes 00:00, from hour - 1 in _parse_epw), but the docstring only says "Days run from 0-23h". EPW irradiance and illuminance are totals over the hour ending at the time in the file, while temperature, humidity and pressure are values at that time, and read_tmy3 labels its records with the end of the hour and warns about it. So the same data read with read_epw and read_tmy3 are one hour apart, which is what #1156 reports.

This PR documents the convention with a warning like the one in read_tmy3, including the one-line shift that labels the records with the time given in the file. Changing the index itself would break existing users; if you would rather do that (perhaps behind a parameter), I'm happy to follow up with a PR. I ran into this while aligning EPW weather for a building simulation side project.

Checked: read_epw on tests/data/NLD_Amsterdam062400_IWEC.epw with coerce_year=2001 gives 2001-01-01 00:00 for EPW hour 1 and 2001-12-31 23:00 for the last record; tests/iotools/test_epw.py passes; flake8 is clean. I'll add the :pull: reference to the what's-new entry once this has a number.

Prepared with AI assistance (Claude); I reviewed it.

[github-actions]

Hey @cdeil! 🎉

Thanks for opening your first pull request! We appreciate your
contribution. Please ensure you have reviewed and understood the
contributing guidelines.

If AI is used for any portion of this PR, you must vet the content
for technical accuracy.

Finally, be sure the PR description includes the PR
checklist,
and complete the items you are able to.