Allow non-RST parsers to substitute the Locale transform

Describe the bug

https://github.com/sphinx-doc/sphinx/commit/79650f5827e146afe3ff50485f8a4bd6755f895d (#4938) added code that adds a line of hyphens under (for example) an admonition's title's text.

However, a line of hyphens is an RST-specific syntax for a heading. It does not indicate a heading in all parsers.

In Markdown, for example, a line of hyphens is a second-level heading https://spec.commonmark.org/0.29/#setext-headings. This can cause the heading level to jump from 0 to 2.

To Reproduce

I can create a test scenario, but the bug should be clear from reading the code, with the above context.

Expected behavior

For Markdown, we can just change the hyphen to an equals sign, which Markdown will interpret as a first-level heading.

For other parsers, I suppose there would need to be hooks for those parsers to convert the string to a heading.

Start by tracing the Locale transform introduced in commit 79650f5827e146afe3ff50485f8a4bd6755f895d (#4938), focusing on where the hyphen line is added for parser output. Compare that behavior with Markdown setext headings and determine how non-RST parsers can provide their own heading representation. Done means Markdown no longer causes the heading level to jump and other parsers have an appropriate extension point.