Wrap all '@' [at] symbols in yaml with single quotes

[weaverryan]

In 2.8, unwrapped @ symbols are deprecated. We need to update that across the docs. I think this is already a standard, but I don't know if we're following it everywhere.

[xabbuh]

I am pretty sure it is because the Sphinx lexer uses the Yaml 1.1 spec and otherwise wouldn't render them with pretty syntax highlighting.

[wouterj]

See #5911 (IIRC, Yaml 1.1 only forbits to use @ as the first character in a string)

[xabbuh]

Tha's true but even the YAML 1.2 spec only mark them as reserved as the first character in an unquoted sring (see http://www.yaml.org/spec/1.2/spec.html#id2774157).