Offset page content

After migrating to the new `sphinx-book-theme` (xref https://github.com/dask/dask-sphinx-theme/pull/54), I noticed when navigating to a specific subsection of a page, the section title is covered up. For example, previously https://docs.dask.org/en/stable/array-creation.html#concatenation-and-stacking would look like:

while with the new theme we have:

(Note the "Concatenation and Stacking" title is covered).

I recall this happening before, even with the old theme, because we didn't account for the width of the top navbar in the Dask theme. This was resolved by https://github.com/dask/dask-sphinx-theme/pull/27. Perhaps we didn't carry that extra CSS over when we switched themes?

cc @jacobtomlinson

Reproduce the covered subsection heading at the linked array-creation.html anchor, then inspect the theme changes in dask-sphinx-theme PRs #54 and #27 for the navigation-offset CSS. Done means subsection headings remain visible below the top navbar when navigating to an anchor.

Written by the indexing model from the issue text.