Document locale identifier casing and parsing

[himanshu748]

Fixes #115.

Document recommended language, territory, script, and variant casing in the locale guide and API. Explain that Locale preserves constructor components while Locale.parse normalizes string identifiers, with mixed-case examples. Numeric territory codes and modifier casing are also covered. No runtime behavior changes.

Validation on Linux / Python 3.12.14 with the project's CLDR 48.2 import:

• Full suite: 7,831 passed, 7 skipped, 2 xfailed, matching the unchanged baseline.
• Core/localedata and API doctests: 151 passed, 1 skipped.
• Guide constructor/parsing examples: 8 passed.
• All applicable pinned pre-commit hooks, Ruff, and git diff --check passed.
• Executable Python AST is unchanged after removing docstrings.
• Strict Sphinx output matches the unchanged baseline's 111 unresolved-reference warnings and 5 pre-existing markup errors. The ordinary warnings-as-errors build has the same 5 markup errors (JavaScript extractor docstring indentation and unresolved targets in dates/dev/messages).

Other Python/OS matrix combinations remain for hosted CI.

Hosted CI and Security workflows currently await maintainer approval.