docs: complete add_js_file and add_css_file documentation

[cnbei]

Purpose

Fill the remaining gaps in the Sphinx.add_js_file() and Sphinx.add_css_file() docstrings reported in #12223:

• Show the CRC32 cache-busting query parameter (?v=) in the examples (added in Append CRC32 checksum to asset URIs #11415 / Sphinx 7.1).
• Add versionchanged:: 7.1 notes for that checksum.
• Explain that these methods only emit the HTML tag; local files still have to land in _static via add_static_dir() (extensions) or html_static_path (site authors).
• Point site authors at html_js_files / html_css_files with html_static_path.

This is a new PR, not a continuation of #14322.

No CHANGES.rst entry (documentation-only).

References

• Incomplete documentation for add_js_file() and add_css_file() #12223
• Append CRC32 checksum to asset URIs #11415

Closes #12223.

AI Disclosure

A Cursor Cloud Agent prepared this change at the requester's instruction.

• Tools: Cursor Cloud Agent. Used to read Incomplete documentation for add_js_file() and add_css_file() #12223, the current add_js_file / add_css_file docstrings, the HTML builder checksum implementation, add_static_dir, and the html_js_files / html_css_files / html_static_path confvals, then draft the docstring additions.
• What is AI-generated: the new paragraphs, example ?v= suffixes, versionchanged 7.1 notes, expanded seealso lines in sphinx/application.py, and this pull request body.
• What is not: no runtime code was changed. The wording was checked against the implementation.
• Improve documentation for add_js_file() and add_css_file() #14322 was read only to avoid duplicating it; this text was written separately.

I have read https://www.sphinx-doc.org/en/master/internals/ai-policy.html.