ensure no page switching + add core concepts

[gingerwizard]

Summary

Adds pages to avoid switching between menus which is both jarring and hard to follow.

Added @tom-clickhouse guide on parts under a new core concepts section.

This is imperfect, we need better dedicated pages vs just importing reference documentation in some places. But progress is more important.

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