Website accessibility

I took the Accessibility Insights plugin in Edge for a testdrive over a sample
of the website, and thankfully it reported very few issues. Making sure we
have good support for screen readers is important so I had a look at fixing the
issues.

* There are interactive elements which lacks labels: the search buttons and
the theme button. The easiest fix here is to add aria-label attributes on
these.

* The version toggles on the featurematrix lacks labels. This turned out
to be a bug in the template, there is a label but it's assembled without
all its parts so the end result doesn't point to the checkbox.

There are a few reports against the docs as well, but I think they mainly fall
into the false positivies category.

The attached patches address the above top two things, which should make the
accessibility report green on most of the site pages.

--
Daniel Gustafsson