ARM: Make parameter names clickable

Someone (@vicky?) suggested on the Kea call:

If we want to link the statements in the ARM the way we did for the BIND ARM, I have the MRs that Petr used. The linking was built on top of a couple of other things - including the category tagging we also did in the ARM. I think these are the Sphinx text roles that we changed style for recently. Nice idea. Not trivial, requires some Sphinx python programming.

The overall goal is to reuse a script that BIND developed. It turned statements (e.g. hosts-database) into clickable links.