[docs] Fix website broken links

[wolfkill]

What changed

This fixes internal website links that currently make the Docusaurus production build fail:

• point Quickstart/Documentation links at the generated current-docs route under /docs/next/quickstart/flink
• point the Kafka comparison architecture link at /docs/next/concepts/architecture
• replace downloads-page links to missing internal release blog pages with GitHub release-note links for the corresponding tags

Root cause

The current docs version is configured with the path next, so links to /docs/quickstart/flink and /docs/concepts/architecture no longer resolve in the generated site. The downloads page also linked to /blog/releases/0.9/ and /blog/releases/0.8/, but those blog routes do not exist in the website tree.

Verification

RED on upstream/main:

• npm run build failed with Docusaurus broken links for /docs/quickstart/flink/, /docs/concepts/architecture/, /blog/releases/0.9/, and /blog/releases/0.8/

GREEN on this branch:

• npm run build completed successfully and generated static files
• git diff --check passed

Note: the build still reports existing broken-anchor warnings for anchors such as /downloads/#filesystem-jars and #using-java-client. Those warnings do not fail the build and are outside this broken-link fix; some overlap with already-open docs anchor work.