WCAG 2.1 site header causes reflow issue

[fstrr]

The site header in the WCAG 2.1 version of the site causes reflow issues when zoomed in or viewed on a phone. Thiis screen grab is from a laptop browser zoomed into 400%:

[kfranqueiro]

This will presumably be resolved once we're ready to generate 2.1 docs with the new build system, given that this does not happen on current 2.2 docs (IIRC @iadawn did some recent work on the header).

#4007 is the first step towards that, but there may be some remaining work afterwards, e.g. filtering working examples for 2.1 output.

[iadawn]

Should do. I have tagged with 2.1 so we can check and close this once we are comfortable and have published the update to 2.1