docs: administration: yaml: document the extensions section

[eschabell]

• Add extensions to the list of supported top-level YAML sections.
• Add an "extensions section" section describing it as a place to keep settings for tooling around Fluent Bit, which Fluent Bit parses and retains but the data pipeline never reads.
• Note that extensions is the only section accepting nested maps, and that nesting elsewhere fails with "variant values are only valid in the extensions section".

Note, update for code changes without corresponding docs PR.

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• administration/configuring-fluent-bit/yaml.md

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[eschabell]

@patrick-stephens ready for review!