rustdoc: should expect and interpret valid Markdown

[projektir]

rustdoc currently expects all Markdown files to start with the % symbol, turning that line into an <h1> heading with the corresponding title. The % symbol is not part of Markdown, and this notation seems unnecessary.

Instead, rustdoc should be able to read #, ##, and so on, as expected, turning them into <h1>, <h2>, respectively, and then correctly numbering the child headings. The first heading in the file can be used for the title and table of contents parenting. This should fix #39850.

This will change rustdoc to additionally process the first # heading (or any first heading?) the way it currently processes the % heading.

I plan to work on this issue, but if anyone knows something about the history of % that could be useful information.

[steveklabnik]

I plan to work on this issue, but if anyone knows something about the history of % that could be useful information.

I don't know why I want to say @cmr may, but maybe?

[Mark-Simulacrum]

I believe this is handled correctly today, closing.