gh-101100: Fix Sphinx warnings about list methods

[koyuki7w]

The list methods have no entries in the document, so removed these references.

Fixed 7 sphinx warnings:

library/collections.rst:784: WARNING: py:meth reference target not found: list.append [ref.meth]
library/collections.rst:784: WARNING: py:meth reference target not found: list.append [ref.meth]
tutorial/datastructures.rst:143: WARNING: py:meth reference target not found: list.append [ref.meth]
tutorial/datastructures.rst:143: WARNING: py:meth reference target not found: list.pop [ref.meth]
tutorial/datastructures.rst:342: WARNING: py:meth reference target not found: list.pop [ref.meth]
tutorial/datastructures.rst:495: WARNING: py:meth reference target not found: list.append [ref.meth]
tutorial/datastructures.rst:495: WARNING: py:meth reference target not found: list.extend [ref.meth]

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

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📚 Documentation preview 📚: https://cpython-previews--127054.org.readthedocs.build/

[vstinner]

LGTM.

List methods are not documented in a way that we can refer to them: https://docs.python.org/dev/library/stdtypes.html#mutable-sequence-types