Final call: Add placeholder passed example to form-field-acc-name

Hey folks,
This PR adds an example of using placeholder to pass 4.1.2 with an input
element. All major browsers currently can use placeholder for creating an
accessible name, if nothing else is provided. This PR adds an example that
demonstrates how to do this correctly. The final call ends on August 26th.
If you have any feedback, leave a review on the pull request:

https://github.com/act-rules/act-rules.github.io/pull/1420

-- 
*Wilco Fiers*
Axe for Web product owner - Co-facilitator WCAG-ACT - Chair ACT-R