split kafka docs

[mshustov]

Get back the structure introduced in #740

[robot-ch-test-poll2]

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[Ryado]

I made some substantial changes and pushed them in the last commit

[Ryado]

Can you have a look @mshustov ?

[Felixoid]

So, the changes from ecf6e77 work in https://github.com/ClickHouse/clickhouse-docs/actions/runs/6589667297/job/17904622695?pr=1513

I force push them back, and we'll merge ClickHouse/ClickHouse#55874 ASAP