Enhance DDLWorker pages: lifecycle overview, system.distributed_ddl_q…

[TheraniAA]

…ueue, fixes

_index.md:

• Add explicit lifecycle paragraph (initiator znode, active/finished children).
• Promote hostname/IP mismatch to its own section with a checklist.
• Add 'Inspecting the queue' section using system.distributed_ddl_queue.
• Add restart-as-recovery action and log-capture step to the 'DDLWorker thread crashed' section.
• Expand 'Too intensive stream of ON CLUSTER' with concrete tradeoffs.
• Replace legacy root with in the config example.
• Fix thread-name mismatch (DDLWorkerCleaner -> DDLWorkerClnr).
• Annotate task_max_lifetime as = 1 week.
• Remove self-referential link and add cross-links to the child page and the ZooKeeper KB page.

there-are-n-unfinished-hosts...md:

• Shorten linkTitle to 'Unfinished hosts'; add weight: 100.
• Lead with system.distributed_ddl_queue as the first-stop diagnostic.
• Fix broken heading nesting (Previous task ... promoted from h4 under Debian/Ubuntu to a sibling h3 under Possible reasons).
• Replace duplicated FQDN rows in MaxDDLEntryID example with three distinct replicas showing one that lags.
• Trim the 15-frame stack trace to the essential frames.
• Remove unrelated SYSTEM STOP/START REPLICATION QUEUES lines.
• Update clickhouse.tech -> clickhouse.com.
• Add 'Still stuck?' escalation section.

I have read the CLA Document and I hereby sign the CLA