Add validators reference

[mbrodala]

Current behavior

There is no overview and documentation of the validators shipped with Extbase.

Expected behavior/output

All validators shipped with Extbase and their options should be documented.

Related existing docs:

• Validation in Extbase
• Validator usage/development in Extbase
• Validator usage in forms

TYPO3 versions

all

Possible Solution

Add a list of all available validators with their options.

Additional context

Especially the basically unknown DisjunctionValidator would benefit from this since it avoids the need to extend existing validators to e.g. optionally allow null values.