Shorten fragment links

[philwebb]

Our existing fragment links are too long and can be shortened now that we no longer publish a single page HTML document. This should be done after #40062 so we can deal with redirects.

Some work will be required to make the asciidoc extensions we use work. Specifically around the code import which currently uses the ID to find source code.

[philwebb]

We're going to keep the fragments as they are because we might want to do a fragment link to h1 elements.