Improve and fix markup code preview rendering (#35777)

1. Add the color on the link to the referenced file, which is the more
likely thing the user wants to click
2. Use monospace font on the SHA
3. Tweak text colors
4. Change SHA link to go to the commit instead of the repo root with
commit filter set
5. Added the repo name to the file link text
6. Fix broken line numbering rendering

web_src/css/markup/codepreview.css

@@ -5,6 +5,7 @@
 }
 
 .markup .code-preview-container .code-preview-header {
+  color: var(--color-text-light-1);
   border-bottom: 1px solid var(--color-secondary);
   padding: 0.5em;
   font-size: 12px;