Enable fixing links for other formats

[noukenolife]

Problem

Currently, it seems link fixes (fix .md to .html) are only applied to the rendered html. So, links in other formats such as LaTex are not fixed.

https://github.com/rust-lang/mdBook/blob/master/src/utils/mod.rs#L142-L161

Example (LaTex)

Before Fix:
$$
\href{./index.md}{link}
$$

After Fix:
$$
\href{./index.html}{link}
$$

Proposed Solution

Applying link fixes for all formats seems too much, so we might be able to at least cover supported format in mdBook such as LaTex.

Notes

No response