[docs] Align documentation with Pulsar 5.0 capabilities and upgrade paths

[lhotari]

This brings the current documentation in line with Pulsar 5.0 and refreshes the 5.0.x versioned docs from that baseline. The release highlights put Scalable Topics alongside performance and reliability improvements for existing workloads, with a clear distinction between upgrading a cluster and adopting new capabilities.

• Add dedicated 5.0.x upgrade guidance covering rollback preparation, configuration defaults, metrics, applications, Functions, plugins, and standalone instances.
• Expand Scalable Topics and v5 Java client guidance while clarifying continued v4 client and existing-topic compatibility.
• Centralize Java dependency setup and Netty alignment, and update authentication, TLS, and security guidance.
• Document broker performance tuning, ZGC and Linux memory settings, monitoring, and system performance testing.
• Refresh deployment, Helm, metadata-store, load-balancing, connector, and contributor documentation.
• Improve navigation, redirects, shared version variables, and table readability, and add automatic release-note links.

I suggest merging this as the documentation baseline and revisiting the details through focused follow-up PRs. That should let us efficiently improve the content from a shared baseline rather than hold the full update while refining every detail.

Validation: yarn build --locale en passed for current and 5.0.x docs, and git diff --check passed. The build still reports existing, unrelated /community/ anchor warnings. The 5.0.x docs match docs/; their sidebar matches the current sidebar with version-specific links resolved.

The versioned documentation refresh is a separate commit from the main documentation update.

Contribution checklist

• Updated the 5.0.x versioned docs and sidebar.