[material-ui] Refine the Sign-in and Sign-up templates

[zanivan]

• I have followed (at least) the PR section of the contributing guide.

Part of #37555

This PR is for refining the Sign-in and Sign-out templates, to be aligned with the new landing page theme.

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👉 https://deploy-preview-41192--material-ui.netlify.app/material-ui/getting-started/templates/sign-in/
👉 https://deploy-preview-41192--material-ui.netlify.app/material-ui/getting-started/templates/sign-up/
👉 https://deploy-preview-41192--material-ui.netlify.app/material-ui/getting-started/templates/sign-in-side/

[mui-bot]

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https://deploy-preview-41192--material-ui.netlify.app/

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[danilo-leal]

Looking great! Thanks for bearing with me on all the reviews 🤝 Last "major" request is to add labels to all the input fields so they're accessible. We can't rely on just the placeholder. This will naturally increase the height of all of the cards, so keep an eye out for mobile scrolling (everything should be fine, though).

[danilo-leal]

Let's ship it! I think the only design change I'd do right now is aligning the "Forgot password" with the "Password" label. Otherwise, specially on mobile, things get a bit all over the place.