Tutorials to training material & redo index page

Conflicts:
	users/index.txt
Tidying editor & web sections
Adding image & sorting heading hierarchy

Conflicts:
	users/index.txt
Updating Python version required
Fixing demo server page
Text fixes & improvements suggested by Mark
Restoring util scripts to user docs
Add section about the Sphinx linkcheck output parsing

- Add warning priority table based on the link type/error code
- Add warning thresholds table marking the build as unstable
Add fixes suggested by Helen and improve threshold table
Delegate external link linkcheck exclusion to sphinx-ignore-links
Merge pull request #269 from sbesson/rebased/develop/linkcheck_parser

Add section about the Sphinx linkcheck output parsing (rebased onto develop)
Merge pull request #271 from sbesson/rebased/develop/external_linkcheck_ignore

Delegate external link linkcheck exclusion to sphinx-ignore-links (rebased onto develop)
Merge pull request #268 from hflynn/user_develop

User docs develop