update xata docs

[divyenduz]

Summary

Xata is moving towards a Postgres platform (while we offer advance features like branching and zero downtime schema changes), we don't have a special Xata client anymore. This PR updates the docs links to point to the latest platform and update the docs to use standard Postgres libraries instead of custom clients.

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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[Oxyjun]

Thanks for this @divyenduz ! I've made some tiny adjustments, but have merged your changes. Thank you!