Clarified the context of the step-by-step process

Also limited the dependency on branching of 'master' to make the example more versatile and consistent with the best practice in this document.
Merge pull request #1742 from svandragt/patch-1

Clarified the context of the step-by-step process
Enhanced tests for regeneration of images and a bugfix for images that were cached in multiple steps
Merge pull request #1761 from NicoHaase/master

[BUG] Enhanced tests for regeneration of images and a bugfix for images that were cached in multiple steps
Use correct config variable name in encrypt_password

Use correct config variable name in encrypt_password
Fixes https://github.com/silverstripe/sapphire/issues/1709
Merge pull request #1812 from willmorgan/patch-1

Use correct config variable name in encrypt_password