Add getting help section to ClickHouse docs
What does this MR do and why?
Adds getting help* section to ClickHouse docs
Adds team name and slack channel to ClickHouse development documentation.
related to #434281 (closed)
MR acceptance checklist
Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.