Use condensed/simplified literal block syntax
Use condensed/simplified literal block syntax

ReStructuredText supports smart handling of its literal block syntax:[0]

> The handling of the :: marker is smart...
> If it is preceded by non-whitespace, the marker is replaced by a single colon.

This commit takes advantage of this syntactic sugar wherever possible.
It also enforces a conventional three-space indentation for all literal
code blocks. This has the added benefit of allowing proper nesting of
multi-paragraph list items (see `.. tip:: On Mac OS...`).

[0]: https://www.sphinx-doc.org/en/master/usage/restructuredtext/basics.html#literal-blocks

zammad/zammad-documentation,zammad/zammad-documentation