Review all `h1` font sizes

The following discussion from !22155 (merged) should be addressed:

• @annabeldunstone started a discussion: (+1 comment)

  @matejlatin is this the size all h1 elements should be? I see it specified for markdown h1s, but there are a lot of variations in other files. Maybe we should create a separate issue to fix these.

Proposal

All h1 elements should be 28px