Add landing pages for Getting Started page

[Blargian]

Summary

Adds landing pages where missing, plus some restructuring where needed.
Example datasets menu to be PRd on main repo.

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/en/integrations/index.mdx

[Blargian]

index.md was previously "Using ClickHouse for Observability" -> /en/observability . Index.md becomes the landing page, I have moved this page to introduction.md at /en/use-cases/observability/introduction and this page gets slug /en/use-cases/observability. Added redirect.

[gingerwizard]

not sure we need the redirect @Blargian we want people to index. This wont break anything.

[Blargian]

We'll need it. It goes from en/observability to /en/use-cases/observability

[gingerwizard]

minor see comments. thanks!