mdbook internal HTML id's conflict with header id's.

[ehuss]

If there is a page with a header of "Rust", then the fragment URL will not work for that section because there is a conflict with the id used by the theme selector.

To reproduce, create a page with ## Rust. If you try to click on the header, the right-pointing quotation mark does not appear. The #rust fragment does not scroll to the section. The same will happen with other sections with the same name as a theme.

This is a problem with the rust-lang/rust linkchecker which requires that all id's are unique.

Perhaps mdbook id's (like the theme buttons) should use more unique id names (maybe mdbook- prefix)?