Document declarative (file-based) configuration

[MikeGoldsmith]

What

The SDK supports declarative configuration (loading the SDK from a YAML/JSON file per the configuration spec), but there is no user-facing documentation for it. Go and Java document this flow; Python should too so it can be recommended for the common "point the SDK at a YAML file" use case.

Scope

• A guide/reference page under docs/sdk/ covering:
  • enabling via OTEL_CONFIG_FILE
  • applying programmatically via the public opentelemetry.sdk.configuration API
  • the file-configuration install extra
  • environment variable substitution
• A runnable example under docs/examples/declarative-config/ (config YAML + app).

Dependencies

• feat(config): support OTEL_CONFIG_FILE in the SDK configurator #5271 — honor OTEL_CONFIG_FILE in the SDK configurator
• feat(config): add public opentelemetry.sdk.configuration namespace #5276 — public opentelemetry.sdk.configuration namespace

Part of the declarative configuration effort tracked in #3631.