Improve help and guidance

[ajparsons]

We want to improve guidance on how to campaign using WriteToThem (enable broader work), to enable that we want to shift the help docs into markdown to make them easier for non-technical people to maintain.

Current tasks:

• Move existing help to markdown
• Revise and approve new copy.
• Flag any broader site language changes suggested by this.

As we're also making broader copy changes, this should include #492 if it's not merged first.