Add a guide for changing export templates

[nathanlesage]

One question that came up often in the past months is "How do I change what my exports look like?" -- so I think we need a guide on how to do that, both for templates and for Odt/docx documents.

[rolfedh]

The following files appear to be interrelated:

• docs/en/core/export.md
• docs/en/advanced/custom-templates.md
• docs/en/reference/markdown-basics.md

What changes did the inquirers intend to make? Which requests were most commonly made?

[Taxicletter]

One small thing is this error:

While many variables are optional, there is one Pandoc variable that needs to be present at all times:

$body$

in docs/en/advanced/custom-templates.md
You also need something like this

\documentclass[11pt]{letter}
\begin{document}


$body$

\end{document}