Broken Link to Ayu theme in The rustdoc book, Section 4.1

[lpnh]

Location

The rustdoc book, Section 4.1 - What to include (and exclude)

Summary

Broken link found at the end of Section 4.1. The link text is "Ayu", which was intended to lead to an ayu.css file previously located in src/librustdoc/html/static/css/themes/. However, following a commit, all CSS files in the themes directory have been merged into src/librustdoc/html/static/css/rustdoc.css. The Ayu theme code is now found in rustdoc.css at L2384-L2574. Suggesting to update the link to point to the new location of the Ayu theme in rustdoc.css.