Search improvement: add landing pages for ClickPipes, MySQL and MongoDB

[Blargian]

Summary

This PR adds landing (table of contents) pages for MySQL, MongoDB and Postgres ClickPipes pages so that we can have a nicer search result for "MongoDB", "Postgres", "MySQL" which will show this overview page.

• index.md pages become the overview
• old index.md becomes ingesting_data.md

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/integrations/index.mdx

[vercel]

@Blargian is attempting to deploy a commit to the ClickHouse Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

The latest updates on your projects. Learn more about Vercel for GitHub.

Project	Deployment	Preview	Updated (UTC)
clickhouse-docs	Ready	Preview	Aug 27, 2025 11:17am

[Blargian]

Preview link: https://clickhouse-docs-yeptq7qby-clickhouse.vercel.app/docs/integrations/clickpipes/postgres/overview

[morsapaes]

Ah, I had something different in mind, @Blargian. What I'd like us to have is a page like this for data ingestion technologies that are supported in more than one way in ClickHouse. Sorry if that wasn't clear! This could then be the heavy hitter in search rank. For each ClickPipe, I think it's fine to just land in the existing pages; adding an index page might add more friction to getting started.

[Blargian]

Discussed with @morsapaes, and added some rules in Algolia. We'll leave the index pages out.