docs: Take advantage of extra level of headings.

The way Sphinx treats the "top-level" adornments is weird. It usually
uses the first top-level adornment as the page title, even if the
top-level adornment is just one "section" out of many (i.e. if the first
section is "Introduction", then it will make the page title be
"Introduction"). This behavior can be overriden by using an explicit
`..  title::` directive to set the title.

Since the Sphinx stylesheet that Clang is currently using ('haiku')
nicely puts the document title at the top of the page in the header,
this weird default behavior was resulting in a redundant "title" in the
body content. Getting rid of this redundant level of headings
effectively "exposes" one more level of heading from the stylesheet to
which now makes the real "sections" more distinct.