Add detailed docstrings for WEATHER_KEYS, POA_KEYS, and TEMPERATURE_KEYS

[Chirag3841]

(x means done)

• Closes Run ModelChain from POA irradiance #943 (documentation portion)
• I am familiar with the contributing guidelines
• Tests added (not required for documentation-only PR)
• Updates entries in docs/sphinx/source/reference (not required for constants docstrings)
• Adds description and name entries in "what's new" file (not needed for small internal doc wording)
• New code is fully documented (docstrings added)
• Pull request is complete and ready for review
• Maintainer: assign labels and milestone
  @cwhanse
  I have done changes for PR1.

[Chirag3841]

@cwhanse
PR-1
PR Title
Add detailed docstrings for WEATHER_KEYS, POA_KEYS, and TEMPERATURE_KEYS

PR Description
This PR improves the documentation of three ModelChain constants added in #943:

1)WEATHER_KEYS

2)POA_KEYS

3)TEMPERATURE_KEYS

Each constant now includes a detailed docstring explaining its purpose, usage, and the data expected by the ModelChain pipeline.

The goal is to improve readability and help new contributors understand how input dictionaries are parsed within ModelChain.
I will add required methods after this (remaining PR).

[cwhanse]

@pvlib/pvlib-maintainer I think adding comments here is enough for now. These constants are not meant to be modified by users. The documentation page for pvlib.modelchain.ModelChain is already a hot mess with all the methods, so I don't see any value to adding these constants.

@Chirag3841 please address the lint issues.

[Chirag3841]

@cwhanse
Changes applied. Please review.

[cwhanse]

#2602 and #2603 are still stacked on top of this PR. We can review them sequentially.