[Task](ui): ux docs - improve and expand Forms article

[franzheidl]

Task Description

Collect possible/necessary improivements to our form docs here:

• add a reference to the w3c forms tutorial (https://www.w3.org/WAI/tutorials/forms/) – it is very good and exhaustive, there is no point in re-creating smth like this. Ours would never be as good and complete
• add some general recommendations on top, just as a quick summary of good practices we could add and that are more tailored to juno-ui components.
  • use unique ids and how (already included?)
  • communicate expectations, no surprises (hints to individual inputs, hints for groups or complex input value requirements such as password complexity)