Update RST documentation to use asset() Twig function

[robfrawley]

The README.md code examples now make use of the Twig asset() function, per #825. We should bring the relevant examples in the RST documentation in-line with this convention. While the prior mentioned PR affected only four README.md lines, I know that their are a few additional examples in the RST documentation, all of which should be consistent, IMHO.

[alexwilson]

Agreed! Have you got this covered or would you like me to have a crack at this?

[robfrawley]

I won't be about to get to it until next week, so if you have time, feel free to give it a go. Otherwise I'll take care of it in the coming weeks. A little crazy busy for me ATM. ;-)

[alexwilson]

I've just resubmitted my PR under #836