Rollup merge of #136784 - yotamofek:pr/rustdoc-remove-buffer-take2, r=GuillaumeGomez

Nuke `Buffer` abstraction from `librustdoc`, take 2 💣

In rust-lang/rust#136656 I found out that the for_html field in the Buffer struct was never read, and pondered if Buffer had any utility at all. `@GuillaumeGomez` said he agrees that it can be just removed. So this PR is me removing it. So, r? `@aDotInTheVoid` , maybe?

Supersedes #136748

Author: GuillaumeGomez