Adds API intro video

Adds an "introduction to the API" video to `/how-to`. Since the page is no longer just Dashboard videos, I've also updated a few other bits of copy to take that into account.
We're going to put these in emails so it'd be nice to link to them directly
(i.e. /features/api/#video). Is there a better way to do that now?

--

*Tim Rogers*
Sales Engineer
GoCardless.com <https://gocardless.com>

020 3397 7527



On Wed, Jul 23, 2014 at 10:31 AM, Philip Harrison <notifications@github.com>
wrote:

> In app/pages/features/api/index.html:
>
> > @@ -103,15 +103,34 @@ <h1 class="u-color-meta u-text-h3 u-text-heading align-btn">We maintain official
> >      </div>
> >    </div>
> >
> > +  <a name="video"></a>
>
> What is this for?:)
>
> —
> Reply to this email directly or view it on GitHub
> <https://github.com/gocardless/splash-pages/pull/80/files#r15278755>.
>