Move content across from alphagov/tdt-documentation

[bravegrape]

Context

Changes proposed

How to review

TO DO

• [x] move content from alphagov/tdt-documentation
• [x] make sure the content has the latest updates from docs
• [x] put all images in a separate docs/source/images folder
• [x] make sure images are referenced correctly from the new location
• [ ] remove fingers
• [ ] write a proper PR description
• [ ] sort out .gitignore
• [ ] fix links referring to old docs folder, for example in CONTRIBUTE.md
• [ ] check that all links still work
• [ ] deploy to PaaS
• [ ] set up Travis to deploy automatically
• [ ] check all content again before archiving, in case other changes have been made