gh-101100: Fix sphinx warnings in tutorial/appendix.rst

[sobolevn]

Warnings before:

/Users/sobolev/Desktop/cpython/Doc/tutorial/appendix.rst:104: WARNING: py:mod reference target not found: sitecustomize
/Users/sobolev/Desktop/cpython/Doc/tutorial/appendix.rst:104: WARNING: py:mod reference target not found: usercustomize
/Users/sobolev/Desktop/cpython/Doc/tutorial/appendix.rst:116: WARNING: py:mod reference target not found: sitecustomize
/Users/sobolev/Desktop/cpython/Doc/tutorial/appendix.rst:116: WARNING: py:mod reference target not found: usercustomize

The problem here is that these module are not documented as .. module::, they are documented as .. index instead https://www.sphinx-doc.org/en/master/usage/restructuredtext/directives.html#directive-index

cpython/Doc/library/site.rst

Lines 112 to 126 in 044b8b3

	.. index:: pair: module; sitecustomize
	After these path manipulations, an attempt is made to import a module named
	:mod:`sitecustomize`, which can perform arbitrary site-specific customizations.
	It is typically created by a system administrator in the site-packages
	directory. If this import fails with an :exc:`ImportError` or its subclass
	exception, and the exception's :attr:`name` attribute equals to ``'sitecustomize'``,
	it is silently ignored. If Python is started without output streams available, as
	with :file:`pythonw.exe` on Windows (which is used by default to start IDLE),
	attempted output from :mod:`sitecustomize` is ignored. Any other exception
	causes a silent and perhaps mysterious failure of the process.
	.. index:: pair: module; usercustomize
	After this, an attempt is made to import a module named :mod:`usercustomize`,

So, I went with :index: inline role. This way once some location is added to their definitions (a header, probably) they will automatically link there.

• Issue: Fix all Sphinx reference warnings in the documentation #101100

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📚 Documentation preview 📚: https://cpython-previews--108750.org.readthedocs.build/

[miss-islington]

Thanks @sobolevn for the PR, and @hugovk for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[miss-islington]

Sorry, @sobolevn and @hugovk, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 3047f09490ae63f25d57efe1d14a9a65d9b5f6db 3.12

[bedevere-bot]

GH-108757 is a backport of this pull request to the 3.11 branch.

[bedevere-bot]

GH-108759 is a backport of this pull request to the 3.12 branch.