docs: fix all Sphinx documentation warnings

[gijzelaerr]

Summary

• Add sphinx.ext.intersphinx with Python docs mapping to resolve ~110 stdlib type warnings (datetime.datetime, pathlib.Path, socket.socket, logging.Logger, struct, etc.)
• Add nitpick_ignore for ctypes types that intersphinx can't resolve (ctypes.Array, ctypes.c_int, ctypes.c_uint, snap7.type.S7DataItem, snap7.type.c_ubyte_Array_65536)
• Add missing doc/API/error.rst and include it in the index, so cross-references to S7ProtocolError, S7ConnectionError, etc. resolve
• Fix malformed multi-line Raises sections in set_fstring and set_string (continuation lines were parsed as separate exception types)
• Qualify cross-references: Tag → ~snap7.tags.Tag, BlocksList → ~snap7.type.BlocksList, TS7BlockInfo → ~snap7.type.TS7BlockInfo, exceptions → ~snap7.error.*
• Replace unresolvable :class:s7.Client / `:class:`s7.Server / :class:s7.AsyncClient`` with code literals (the s7 package isn't in the Sphinx docs)
• Replace unresolvable :meth: references in s7commplus docstrings with code literals

Before: 139 warnings. After: 0 warnings. Build passes with sphinx-build -W -n.

Test plan

• sphinx-build -W -n -bhtml doc/ doc/_build succeeds with 0 warnings
• uv run pre-commit run --all-files passes
• No new mypy errors introduced

🤖 Generated with Claude Code