Decompiler docs: Specify DocBook XSL Stylesheets V1.78.1 instead of "current"

This makes building the docs via gradle cleanBuildDecompilerDocumentationHtml and gradle buildDecompilerDocumentationHtml much more consistent. I still got some slight differences (which I have included in this PR) but they are a lot less significant than what happens when "current" is used.

$ xsltproc --version
Using libxml 20910, libxslt 10134 and libexslt 820
xsltproc was compiled against libxml 20910, libxslt 10134 and libexslt 820
libxslt 10134 was compiled against libxml 20910
libexslt 820 was compiled against libxml 20910

This is intended to help with #2810, where I needed to make actual doc changes and don't want the noise from this showing up in that PR. (For now I've included the same commit there, though).