docs: process code blocks similar to Markdown

docs: process code blocks similar to Markdown

Wrap pre-formatted example code in <code> elements. This works
similar to Markdown[1] code blocks[2]:

Every line indented with at least 2 spaces is considered a code
block and gets wrapped in <pre> and <code> tags.

Look at the documentation for e.g. virStreamSend for before-and-after
effects.

[1] http://daringfireball.net/projects/markdown/
[2] http://daringfireball.net/projects/markdown/syntax#precode