Rollup merge of #109187 - clubby789:askama-source, r=GuillaumeGomez

Render source page layout with Askama

~~I was looking at making `code_html` render into the buffer instead of in advance, but it turned out to need a pretty big refactor, so starting with rearranging the high-level layout.~~
Found another approach which required much less changes

cc #108868

Author: Dylan-DPC