docs: harmonize headings and titles

docs: harmonize headings and titles

We're pretty insonsistent in what the headings and titles are, especially
compared to what the articles are listed as in the sidebar. Let's
harmonize this.

There's a notable exception for meson.html, where the sidebar uses a
short-hand form that makes sense in the sidebar, but not in the article
due to the visible context being different.

Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>