Update stale docs references to pkg_resources as active module

[veeceey]

Summary

Addresses #5179. Following the deprecation of pkg_resources completed in #5173, several non-historical documentation pages and docstrings still reference pkg_resources as though it is an actively maintained module. This PR updates those remaining references:

• docs/setuptools.rst: Rewrite the "Transitional Note" section to reflect that pkg_resources-style namespace packages are deprecated, with a note directing users to PEP 420 implicit namespaces
• docs/development/index.rst: Remove mention of pkg_resources module familiarity and reference manual (the reference manual no longer exists in the main docs)
• docs/userguide/distribution.rst: Replace pkg_resources.parse_version() with packaging.version.parse() and update examples to use PEP 440-compliant version strings
• docs/userguide/extension.rst: Reference importlib.metadata as the primary API for EGG-INFO metadata access, with pkg_resources as the predecessor
• docs/userguide/package_discovery.rst: Fix pkg_resource typo (missing s) in two places, fix pkgutils typo (should be pkgutil), and add a .. deprecated:: admonition to the Legacy Namespace Packages section clarifying these are no longer supported
• setuptools/dist.py: Replace pkg_resources.require() format reference with PEP 508 in the install_requires docstring

Test plan

• Verify the RST renders correctly (no broken cross-references or formatting) - confirmed via ReadTheDocs preview build
• Confirm all pkg_resources references in non-historical docs now either point to the modern replacement or are clearly marked as deprecated
• No functional code changes -- documentation and docstring only

[veeceey]

Hi maintainers, friendly ping on this PR. It's been open for about 8 days without any review activity. Would appreciate any feedback or direction when you get a chance. Thank you!