docs: generate reproducible ids

With this parameter, docbook will generate the same id's when generating the same docs. This is helpful to get bit-by-bit 'reproducible' output, which makes auditing the package easier.

I made and tested the change to the automake build system.

Likely a similar change will have to be applied to cmake (possibly with the --stringparam parameter to xsltproc?), but I don't currently have my machine set up to test that.