Move content across from alphagov/tdt-documentation

[bravegrape]

Context

Changes proposed

How to review

TO DO

• move content from alphagov/tdt-documentation
• make sure the content has the latest updates from docs
• put all images in a separate docs/source/images folder
• 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

[huwd]

This needs another review and an update I think, I'll see if I can find time.
bringing the docs over and removing broken / dead TDT links sounds good - but I think we need to update the actual guidance that comes across, I'm seeing some PaaS references and lots of ruby v2.x stuff,

I'll go through line by line but I think this is a minor update after v4