Kafka structure update

[lesandie]

Summary
This PR restructures the Kafka KB section to make navigation easier and updates named-collection guidance for modern Kafka configuration workflows.

What Changed

1. Kafka section reorganization
   Kafka articles were regrouped into thematic sections but content and text was not touched.

01-fundamentals
02-consumption-patterns
03-schema-formats
04-operations-troubleshooting
Added _index.md files for each new section to improve hierarchy and discoverability.

2. Kafka landing page update
   Updated section title from Kafka to Kafka engine.
   Added a dedicated librdkafka changelog block on the Kafka landing page.
3. New provider-focused configuration page
   Added:

01-fundamentals/config-by-provider.md
Moved provider-specific Kafka connection/auth examples there (MSK, self-hosted, inline certs, Azure Event Hub, Confluent/Google Cloud), while keeping the core librdkafka page focused on base settings.

4. librdkafka settings page improvements
   Moved:

altinity-kb-adjusting-librdkafka-settings.md into 01-fundamentals
Updated the page to include:

SQL equivalents for XML named collections
kafka.* subsettings in CREATE NAMED COLLECTION
consistent collection names (kafka_preset1, kafka_preset2)
verify/drop examples using system.named_collections
Also updated cross-links so the reading order is clear:

first general librdkafka settings
then provider-specific config page

[lesandie]

Hi please @BorisTyshkevich / @realyota Can you have a look?