docs: clarify the purpose of reporting hooks

[otosmane]

The hook reference lists collection event notifications under Reporting hooks, but does not explain the distinction between reporting events to plugins and displaying results to users. This makes collection hooks such as pytest_collectstart difficult to find when browsing by session phase.

Add an explanation of reporting hooks and reciprocal links between the Collection hooks and Reporting hooks sections. The explanation follows the maintainer guidance in #12658. This change addresses the remaining explanatory gap without moving hooks between categories.

Validation:

• Built the complete documentation with sphinx-build -j 2 -W --keep-going -b html doc/en doc/en/_build/html on Python 3.12; the build succeeded.
• Checked the generated HTML: both new section anchors exist, and the linked hook names resolve within the reference page.
• git diff --check passed.

Prepared with Codex assistance and reviewed by the submitting contributor. The commit includes a Co-authored-by trailer. This is a small documentation clarification; no runtime behavior changes.