check (and warn) for multiple H1 headers

[drammock]

add a quick check for whether there are multiple top level sections in the page, and emit a warning to tell people to nest all headers under one title header

Originally posted by @choldgraf in https://github.com/pydata/pydata-sphinx-theme/issues/1078#issuecomment-1381750827

The rationale for adding this check/warning is that having multiple H1 headers on one page breaks the ability to have custom titles in the TOC, and also there is an unofficial consensus in the accessible design community that each page should have only one H1 header.