YAML introduction docs update (user suggestion)

[eschabell]

Raising issue here to capture user suggestion from online forum: https://fluent-all.slack.com/archives/C0CTQGHKJ/p1759792361290489?thread_ts=1759750481.930109&cid=C0CTQGHKJ

_Suggested change:

ref: https://docs.fluentbit.io/manual/administration/configuring-fluent-bit/yaml

Before you get started
YAML has become essential in a cloud ecosystem. To minimize friction and provide a more intuitive experience for creating data pipelines, users are encouraged to transition to YAML. The YAML format enables features, such as processors, that aren't possible to configure in classic mode.
NOTE: Fluent Bit traditionally offered a classic configuration mode, a custom configuration format that's phasing out. While classic mode has served well for many years, it has several limitations. Its basic design only supports grouping sections with key-value pairs and lacks the ability to handle sub-sections or complex data structures like lists._

[esmerel]

I'm about to make some updates that will turn https://docs.fluentbit.io/manual/administration/configuring-fluent-bit/classic-mode into a real page rather than just a bunch of cards, I can use this as part of it and take care of this user's suggestion.

[eschabell]

Tagging in some credit for @fulldeck (github user) for the contribution!