Document yaml support in configuration

[lecaros]

This is to document yaml support provided by https://github.com/fluent/fluent-bit/pull/4621

• [x] Separate yaml and classic mode. #739
• [x] Add presentation of configuration modes (classic, yaml).
• [x] Add configuration file section.
• [ ] Add Input samples.
• [ ] Add Filters samples.
• [ ] Add Output samples.
• [ ] Add multi pipelines samples.