doc: Clarify conftest.py hooks unavailable during pytest_addoption

[FazeelUsmani]

Fixes #13304

The documentation in "Writing hook functions" incorrectly suggested that hook implementations in conftest.py files are accessible during pytest_addoption() execution. This led to confusion when developers tried to use conftest hooks to provide default values for plugin options.

Changes

• Updated the example in the "Using hooks in pytest_addoption" section to show a third-party plugin instead of a conftest.py file
• Added a clear warning note explaining that conftest.py hooks are not available during pytest_addoption() because conftest files are discovered and loaded after plugin initialization
• Documented which plugin types ARE available during pytest_addoption():
  • builtin plugins
  • plugins explicitly loaded with -p on the command line
  • installed third-party plugins (via setuptools entry points)
  • plugins specified via the PYTEST_PLUGINS environment variable
• Added cross-reference to the plugin discovery order documentation

Testing

• Documentation-only change, no code behavior modified
• Verified RST syntax is correct
• Confirmed all cross-references are valid
• Changelog entry created: changelog/13304.doc.rst