Import markdown docs from stb-tester/docs/

I have moved the "Getting started" article from the stb-tester.com
(website) git repo to the main stb-tester repo, to keep the
documentation closer to the source code.

This also publishes the "OCR tips" and "Backwards compatibility notes"
articles that were previously only available in the stb-tester git repo.
video-capture.md: source_pipelines for GStreamer 1
make release: Also regenerate docs.html & stbt.html

docs.html needs regenerating for the version number in the "release
notes" link text.
make release: Grab release notes from `stb-tester/docs`
0.20 release notes & man page

This commit is the result of running `make release && make` and
committing the changes (after discarding some undesired formatting
changes to stbt.html -- I still need to work on that).
Merge branch '0.20' into gh-pages

* 0.20:
  0.20 release notes & man page
  make release: Grab release notes from `stb-tester/docs`
  make release: Also regenerate docs.html & stbt.html
  video-capture.md: source_pipelines for GStreamer 1
  Import markdown docs from stb-tester/docs/