Move documentation to use sphinx.ext.autodoc

[r266-tech]

This contribution advances #567 by moving duplicated API-reference text into source docstrings and rendering the requirements and pylock modules through Sphinx autodoc.

The review rework follows the module-level automodule pattern established by #572 instead of listing individual classes and fields in the RST files. The pylock module docstring preserves the frozen, keyword-only dataclass contract, while class docstrings carry the field semantics without duplicate undoc-members rendering.

The changes remain limited to docs/requirements.rst, docs/pylock.rst, src/packaging/requirements.py, and src/packaging/pylock.py.

Verification:

• uvx nox -s lint
• uvx nox -s docs (strict HTML and LaTeX builds; 418 doctests, 0 failures)
• git diff --check

[brettcannon]

This isn't following the practices used in the rest of the repo for documentation even when autodoc is used.

[brettcannon]

This :ivar stuff isn't used anywhere else and it pollutes the docstring such that help() becomes less readable.

[r266-tech]

Reworked this to follow the module-level automodule approach used in merged #572 instead of enumerating individual classes and fields in the RST files.

I also kept the pylock frozen/keyword-only data-model contract in the module docstring and removed undoc-members so field descriptions are not rendered twice. I ran uvx nox -s lint and uvx nox -s docs; both passed, with the docs session completing strict HTML and LaTeX builds plus 418 doctests.