docs: "Getting started" documentation for stbt >= 0.19

This is the documentation that is published at
http://stb-tester.com/getting-started.html -- the source used to be at
https://github.com/drothlis/stb-tester.com but I think that it belongs
closer to the stb-tester sources themselves.

I moved it here, updated it to GStreamer 1.x, and rearranged it to focus
on installing from pre-built packages, avoiding GStreamer-specific
knowledge (in favour of telling people what pipeline to use for each
video-capture device we recommend) until you actually need it to debug a
problem, and removing information that we already provide provide
elsewhere (such as http://stb-tester.com/video-capture.html and
http://stb-tester.com/infrared.html).

In time I hope to move more of the documentation from
https://github.com/drothlis/stb-tester.com to here.
docs/ocr.md: Use local copy of image

So that the whole article + image can be copied over to the website
(otherwise stb-tester.com/../tests/ocr/UJJM2LGE.png won't be a valid
image URL).