[DOCUMENTATION] Website Facelift #302

I want to do some changes on the website that include:

1. Move the docs into the main repo
   From https://github.com/testdeck/testdeck-site and https://github.com/testdeck/testdeck-jekyll-theme into the docs/ in https://github.com/testdeck/testdeck
   Hopefully this will allow features and docs to be updated simultaneously from a single PR.
2. Make light/dark homepage themes
   Currently we roll with a light theme, but a lot of developers fancy dark. Adding some "@media preffered dark" CSS to dual style the homepage...
3. Add some more code and a bit less text blocks on the homepage
   Currently when the homepage is opened the code block is out of sight:

@suite class TestSuite {
  @test
  someTest() {}
}

4. Make the site a bit more mobile friendly
   Currently on a mobile phone, there are two hamburger buttons (top right, and a little lower to the left), and there is padding that shrinks the content to 4 to 5 words per line.

WIP
There is some content in the docs/ currently that will be replaced with the WIP from here:
https://github.com/testdeck/docs

A preview is available here:
https://testdeck.github.io/docs/

All the content from the existing site should be preserved except for the landing page.