std domain: parse conjoined option placeholders

[lawrence3699]

Purpose

Fix std-domain option parsing for conjoined angle-bracket placeholders such as -S<value>.

Previously, the parser treated the whole string as the option name, so the directive registered -S<value> instead of -S. This keeps the placeholder in the argument portion, and extends :option: resolution so references like -S<other> resolve through the registered option stem.

Validation:

• .venv/bin/python -m pytest tests/test_domains/test_domain_std.py -q
• .venv/bin/python -m pytest tests/test_directives/test_directive_option.py tests/test_domains/test_domain_std.py -q
• .venv/bin/ruff check sphinx/domains/std/__init__.py tests/test_domains/test_domain_std.py
• .venv/bin/ruff format --check sphinx/domains/std/__init__.py tests/test_domains/test_domain_std.py

References

• Closes Option directive (std domain) does not recognize options with conjoined values #14323