Improve documentation for add_js_file() and add_css_file()

[dhruvildarji]

Summary

Closes #12223.

This PR improves the documentation for Sphinx.add_js_file() and Sphinx.add_css_file() by addressing all five items raised in the issue:

1. Updated examples with CRC32 checksums — Example output comments now show the ?v=a1b2c3d4 query parameter that Sphinx appends to local asset URIs since version 7.1.

2. Added versionchanged:: 7.1 notes — Both methods now document that a CRC32 checksum is appended as a query parameter to local asset URIs for cache-busting purposes.

3. Hint on copying files to _static — Added a note explaining that extension developers should use add_static_dir() to register directories, while site authors can use html_static_path.

4. Mention of html_js_files/html_css_files for site authors — Added a tip suggesting that site authors may find it easier to use these config values rather than writing an extension.

5. Override note with Regression: CSS file added by extension cannot be overridden by users anymore #12096 caveat — Added a note that files registered by extensions can be overridden by placing a file with the same name in html_static_path, with a caveat linking to Regression: CSS file added by extension cannot be overridden by users anymore #12096.

Test plan

• Documentation-only change, no functional code modified
• Verify the reStructuredText renders correctly in a local Sphinx build

[gastmaier]

I think the overall description could be simpler.
add_js_file always adds the .js file to the .html, and suffixes it with the checksum if it is available.

For example, app.config.html_static_path.append + add_js_file at 'builder_inited' will generate the checksum, while add_js_file at 'builder_inited' and copy_asset at 'build_finished' will not suffix with the checksum, because the generation step already happened.
For both, the .js script tag are added to the html page.

See for reference analogdevicesinc/doctools@42be7a6