Summary

Adds a page for Projections.

• Own simple example using NYC taxi dataset as an introductory example.
• Tom's blogpost https://clickhouse.com/blog/clickhouse-faster-queries-with-projections-and-primary-indexes into a more detailed example.
• Moves section on "when to use projections" to this doc
• We will remove the examples from ALTER PROJECTION page to make it more dry reference and link to this page from there.

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