Update PEP 561 packaging guidance

[nightcityblade]

Fixes #16551.

Update the PEP 561 packaging guidance to:

• explain that setuptools 69 and later automatically include .pyi and py.typed files inside package directories
• recommend pyproject.toml for new projects
• identify the existing setup.py, package_data, and MANIFEST.in examples as compatibility guidance for older setuptools versions

This is a documentation-only change, so no runtime test was added.

Verification:

• .venv/bin/sphinx-build -n -d /tmp/mypy-docs-doctree docs/source /tmp/mypy-docs-out --color -W -bhtml
• .venv/bin/pre-commit run --files docs/source/installed_packages.rst --show-diff-on-failure