docs: Make sure all elements that need it have an id tag.

docs: Make sure all elements that need it have an id tag.

When generating HTML it is useful if every element that can be referenced
has a stable id. If it doesn't a random one is generated which makes it
harder to link to parts of the manual on the website. It also generates
spurious diffs. Explicitly add an id tag for the sect2 and sect3 elements
in dh-manual, a unique id for each legalnotice element and for each
FAQ question and answer.