Update optional docs

[emteknetnz]

• Change "Silverstripe elemental" top level title to "Elemental"
• Screenshots on https://docs.silverstripe.org/en/5/optional_features/elemental/basic_setup/ doesn't show
• userforms showing some readme level stuff code-of-conduct, contribution etc - https://docs.silverstripe.org/en/5/optional_features/userforms/

PRs

• https://github.com/silverstripe/silverstripe-elemental/pull/1165
• https://github.com/silverstripe/doc.silverstripe.org/pull/296

[GuySartorelli]

PRs merged and look good on the site