Enhance form documentation for form default reset behavior

[MaxwellCohen]

Summary

Documents React’s default form-reset behavior when using the function action / formAction props, and explains how to preserve field values when you don’t want that reset. Applying @rickhanlonii's feedback from #7795 and building off of work @aurorascharff recentlly did.

• Adds default reset behavior in Caveats, the action prop section, and a new Troubleshooting entry (“Why does my form reset when I use an action?”). React resets uncontrolled fields after a successful action, matching browser <form action="..."> behavior (including before JS loads).
• Adds “Preserve form values after submission” with a Sandpack example: call e.preventDefault() in onSubmit, then run the action manually inside useTransition while keeping the action prop for progressive enhancement.
• Adds a DeepDive for finer control: requestFormReset from react-dom, and returning submitted FormData from server actions to restore values via defaultValue (with useActionState).
• Updates the multiple-submission-types example to a draft/publish scenario that shows how controlled state and key/defaultValue keep textarea content after “Save draft.”

Closes #8397

Alternative to #7795 and #8465

[github-actions]

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