Documentation: PostSwitchToDraftButton editor component

[colorful-tones]

What? & Why?

Addresses one item in #60358

Adding documentation to existing editor components can help with any of the following:

• encourages knowledge sharing and quicker onboarding for future devs
• supports maintenance and troubleshooting
• mitigates risk

How?

Add a JSDoc comment to the PostSwitchToDraftButton component and run npm run docs:build to populate the README with the newly added documents.

[colorful-tones]

@ntsekouras this particular one was marked with // TODO: deprecate... So, it is likely this PR should not be merged. Please verify and I'll close. Thanks!

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[ntsekouras]

It doesn't hurt to add the docs even if it's quite late 😄 . Let's merge and will open a quick PR for deprecating this. Thank you!