Add examples to KTextbox documentation

[nucleogenesis]

Overview

@GarvitSinghal47 pointed out that some of our components' docs could be improved with some examples, starting with KTextbox.

Not all variations need to be applied, however, a handful of the props' examples would be a great improvement.

Acceptance criteria

The following examples can be made initially and further updates to component docs can be discussed afterward:

• invalid and invalidText -- this will need to have a note to the user regarding what is and isn't valid so that they can test it out
• disabled
• number min max
• textarea
• label

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I think that going beyond 5 examples might ultimately make the page less usable/readable overall but once these are in place we can discuss further.

[GarvitSinghal47]

@nucleogenesis feel free to assign me this issue.