Add user help documentation

[robbieaverill]

Add user help docs to allow CMS editors to understand how to add and manage blocks in a website using elemental and this module.

Tasks

• [ ] Create documentation
• [ ] Add screenshots
• [ ] Add to SS4 version of userhelp.silverstripe.org (github.com/silverstripe/silverstripe-userhelp-content)

Acceptance criteria

• [ ] Docs exist in docs/en/
• [ ] Docs are written in markdown
• [ ] Screenshots represent the latest state of the blocks
• [ ] Documentation is added to userhelp

[robbieaverill]

Closed in favour of https://github.com/dnadesign/silverstripe-elemental/issues/139