Fix Sphinx cross-reference warnings

[zain-asif-dev]

Description

Fixes the reference target not found warnings reported when building the documentation.

• Enabled sphinx.ext.intersphinx and mapped it to the Python standard library docs, so references to types like pathlib.Path, logging.Logger, re.Pattern, etc. resolve.
• Corrected several docstring cross-references that used a bare class or method name where a fully qualified name is required for the reference to resolve wherever the docstring is rendered. In particular, FileSystemEventHandler.on_any_event (and similar methods) are inherited without their own docstring by classes in watchdog.tricks, so a bare :class:FileSystemEvent`` in the base docstring fails to resolve once it is rendered under a different module.
• Added docstrings to DirectorySnapshot.isdir, mtime, and size, which were referenced from another docstring but were otherwise undocumented, and fixed :func: roles that should have been :meth:.
• Fixed a reference to Observer.schedule in quickstart.rst to point at the method's actual defining class, BaseObserver.

A few references can never be resolved and are listed in nitpick_ignore with an explanation:

• A type hint that shows up unqualified (Path) because annotations are stored as strings due to from __future__ import annotations.
• watchdog.utils.bricks.SkipRepeatsQueue, an internal base class that isn't part of the public, documented API.
• The platform specific observer implementations (inotify.InotifyObserver, fsevents.FSEventsObserver, kqueue.KqueueObserver, read_directory_changes.WindowsApiObserver), since only one of these can ever be importable on a given platform.

Fixes #898

Testing

• tox -e docs (runs sphinx-build -aEWb html docs/source docs/build/html, which now succeeds with zero warnings)
• python -m sphinx -n -T -b man docs/source <outdir> (nitpicky mode) also succeeds with zero warnings
• pytest (full suite): 170 passed, 10 skipped, 2 xpassed

[BoboTiG]

That's nice, thanks @zain-asif-dev.

Could you also add a CI job to catch future regressions?

[zain-asif-dev]

Good idea. The existing docs CI job already runs sphinx-build with -W (warnings as errors), but not in nitpicky mode (-n), and it turns out these particular reference warnings are only emitted in nitpicky mode. Without -n, sphinx-build silently renders unresolvable :class:/:meth: references as plain text instead of warning, so the previous CI wouldn't have caught this regression (or future ones).

Added -n to the existing sphinx-build command in tox.ini so the current "Build the documentation" CI step now catches this. Verified it fails with 37 warnings on the code before this PR's fix, and passes cleanly with it.