Unfortunately, we now need to know which .html file has which sections
to link to the correct one in help.fish.
So this script helps extract the sections from pre-built docs. It's
not supposed to be run at build time because
1. These change rarely.
2. We should link to the correct document even if the user doesn't
have the docs built.
And before anyone mentions it: This does *not* parse html with regex.
This "parses" the restricted subset of "class followed by href without
embedded quotes" that sphinx uses here in practice.
