Restructured docs

- Merged HOWTO into README
- Defaulting to install the workflow template
- Moved manual setup to "advanced" section futher down
- Separated out "Usage" chapter
- Added note about reminder task
- Added screenshots
- Compressed "Terminology" content

I think longer term we should separate out the docs
for users ("Usage" chapter), and tailor these towards
the template workflow. I suspect most scenarios will
be covered by that workflow, and for anything else
you can take that separate doc as a starting point.

Also, if we move all documentation to /docs
(leaving only links and a general overview in README),
the docsviewer module can pick it up and put it
onto cwp.govt.nz more easily.

Note this also incorporates some docs changes from @oddnoc
from his previous pull requests.