Create /diff/ page with short explanation of what diffs are and how they work

[rufuspollock]

Create /diff/index.md page (can copy and paste about page).

Content includes:

• Link to specific diffs
• Explain what diffs are and why they are useful
  • We may also want to reuse iamge and content from https://www.opendemocracy.net/uk/jonathan-gray-tommaso-venturini-rufus-pollock/making-climate-negotiations-public

When complete change Read more on front page re diffs to this page i.e. to link to /diff/