The CSS for the docs homepage (docs.rust-lang.org) was using the wrong
URL for the body font, resulting in the fallback serif font being used,
instead of the desired Source Serif Pro fonts.

…int values is computed internally.

doc: use correct body font URLs

The CSS for the docs homepage (docs.rust-lang.org) was using the wrong
URL for the body font, resulting in the fallback serif font being used,
instead of the desired Source Serif Pro fonts.

(It's worth noting that the CSS for rustdoc's API generation got these URLs right.)

Changed reference style in dbg macro docs.

# Description

A continuation of Pull Request rust-lang#59528 :
- Fixed method of referencing and adjusted the references as suggested by @lzutao

…, r=frewsxcv

Add book.toml with title to unstable-book doc

Adding a title to the unstable book based on rust-lang#59554