[low quality ai submission] docs: add directory plugin discovery example

[starx710]

The introductory examples show a single-module host and entry-point plugins, but do not show how a host can discover plugin files in a directory. Add a runnable greetings example between the complete example and the real-world references, with separate host, hookspecs and plugin modules.

The host uses pkgutil.iter_modules() and importlib.import_module(), sorts module names before registration, and skips subpackages. The documentation explains that discovery belongs to the host, how registration order relates to ordinary hook call order, and when entry-point loading is appropriate. Adding another plugin file requires no host changes. Pluggy's implementation and public APIs are unchanged.

Refs #258. This addresses the directory-discovery example requested in the issue; broader documentation restructuring is outside this change.

Validation on Windows / CPython 3.12.13:

• Full suite: 221 passed, including two new subprocess tests exercising the documented command, output order, discovery of an added plugin and skipping subpackages.
• All configured pre-commit hooks pass.
• Strict Sphinx HTML build passes with -W and the Towncrier draft enabled (Sphinx 7.4.7); no warnings are suppressed for this change.

OpenAI Codex assisted with implementation and local validation. No independent human review is claimed.