Allow "valid" placeholders for form inputs

[mauromascarenhas]

Proposed changes

With the new proposal of "CSS only" floating labels, it was not possible to use placeholders as "valid" input hints. Therefore, this PR aims to force the users to provide only a single white space (" ") for placeholder attribute in floating label inputs, or the labels will be "always active", allowing "useful placeholders" to be displayed (see screenshots below).

Summary:

• Inputs with floating labels must have a single white space (" ") as placeholder value;
• Inputs with placeholder values other than a single whitespace will always render their respective labels in "active" state;
• Updated docs so as to make it clear and reflect changes 😄.

Screenshots (if appropriate) or codepen:

Previous behaviour

Input labels overlaps "useful" placeholder values.

Current behaviour

Input labels are corectly rendered when "useful" placeholder values are provided.

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to change).

Checklist:

• I have read the CONTRIBUTING document.
• My commit messages follows the conventional commit format
• My change requires a change to the documentation, and updated it accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[wuda-io]

Cool! Thanks for the clear documentation 👍