Offset page content

[jrbourbeau]

After migrating to the new sphinx-book-theme (xref #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 #27. Perhaps we didn't carry that extra CSS over when we switched themes?

cc @jacobtomlinson