LaTeX template for a technical reference manual / user guide?

You may find the refman class a good starting point; it's designed to be an all-in-one package for technical reference manuals.

Having said that, I use memoir for my technical manuals. It's very customisable, and many people have published such customisations, for example, these chapter styles. The last manual I wrote used a slightly modified version of one of these chapter styles, along with a slightly tweaked page style to give larger margins for handwritten and margin notes.

The manual for memoir is widely regarded as a great document to read in its own right, having good information on typesetting in general, and defining useful terminology in a way that's easy to read.

I don't have a empty document to put here right now, but start with a article

\documentclass[10pt,a4paper]{article}

Then include some packages (What packages do people load by default in LaTeX?)

\usepackage[utf8]{inputenc}
\usepackage[english]{babel}
\usepackage[english]{isodate}
\usepackage[parfill]{parskip}

Then include some tables.

\pagenumbering{Roman}
\tableofcontents
\newpage
\listoffigures
\newpage
\listoftables
\newpage
\pagenumbering{arabic}

Then you start to write...

/Have fun