index.md: Remove unused #header css

The default layout (which this page uses) doesn't have a #header element
(unlike the blog & article layouts).
Makefile: Remove unused pandoc variables
Rename "Developer documentation" to "Documentation"

And rename the documentation index page from "developer.html" to
"docs.html", with a redirect from the old URL so as not to break
bookmarks & google results. This uses the "jekyll-redirect-from"
plugin, which in theory will work on GitHub Pages:
https://help.github.com/articles/redirects-on-github-pages
Change "support@stb-tester.com" to "sales@stb-tester.com"

To avoid confusion with the public mailing list.
Rearrange docs index page

Eventually I want this to be more book-like, like the git book, with
a logical order in which to read things, plus clear reference sections.
What devices can I test with stb-tester?

The images are created using OmniGraffle, a Mac-only (sorry!)
proprietary (sorry!) program.
Update 'stbt batch' example report

This is the result of running:

    cd runner_example &&
    stbt batch report index.html

with stbt 0.19.1-34-g7d8c75b3.