[Form Validation] If length is deprecated, remove from docs

[pzanitti]

Expected Result

All instances of length in the docs should have been renamed to minLength.

Actual Result

Validation Rules no longer mention length as a rule. No examples use length.
https://semantic-ui.com/behaviors/form.html#validation-rules
https://semantic-ui.com/behaviors/form.html#/examples

length is still a possible validation rule when configuring the prompts:
https://semantic-ui.com/behaviors/form.html#form-prompts

[lubber-de]

The community fork Fomantic-UI removed the deprecated length rule by fomantic/Fomantic-UI#2087 and the remaining form prompt setting in the docs by fomantic/Fomantic-UI-Docs#307