[docs][joy-ui] Refine the Sign in template

[zanivan]

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

Refining the Messages template, aiming to remove most of the custom styles and overrides, making it closer as possible to the default theme.

Part of #38582

https://deploy-preview-38942--material-ui.netlify.app/joy-ui/getting-started/templates/sign-in-side/

[mui-bot]

Netlify deploy preview

https://deploy-preview-38942--material-ui.netlify.app/

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Generated by 🚫 dangerJS against 5428060

[danilo-leal]

Looking good! Dropping in just a few details!

[oliviertassinari]

We removed the component="h1" but didn't add it back. It's best for SEO for the developers who will use this. https://app.ahrefs.com/site-audit/3524616/106/data-explorer?columns=pageRating%2Curl%2Ctraffic%2ChttpCode%2Cdepth%2Ch1%2Ch1Length%2CnrH1%2Ccompliant&filterId=1337ec32524c0ae0a368c1dc8c7031c8&issueId=c64d9d0b-d0f4-11e7-8ed1-001e67ed4656.

Fixed in 8770d34