Docs: add "code" to Headers (and index list)

[hasezoey]

Summary

(continuation of #12145 (comment))

This PR adds code blocks to the headers and the index list:

Headers

before:

after:

Index List

before:

after:

PS: yes this change does not screw up any anchor ids or click-ablility

[vkarpov15]

Thanks!

[Uzlopak]

Hmm actually it looks counter intuitive. I would actually not expect a hyperlink

[hasezoey]

I would actually not expect a hyperlink

for what, the header or the index list, or both? i actually think having code as code is standard across some documentation projects

[Uzlopak]

The header is somehow ok. But the menu looks weitd

[hasezoey]

@Uzlopak do you think the index list would look better with text-decorations: underline?
(or maybe only the mentioned rule when hovered?)