Easier navigation within reports

[benbc]

Both the Vaccine Coverage and SRO Measures reports have tables of contents to allow readers to find the bits they are interested in. They are a bit clunky to use because they require the reader to scroll up to the top of the report -- but not quite the top because there is preamble content before the ToC.

We should think about whether we can make it easier for readers to navigate within a report. Foldable sections? Multi-page reports? Persistent ToC in the margin of the page?