Update C++ implementation status

[marcalff]

Document that the following C++ features are now implemented:

• [CONFIGURATION] File configuration - exemplar filter opentelemetry-cpp#3837
• [CONFIGURATION] File configuration - implement distribution opentelemetry-cpp#3833

[marcalff]

To @open-telemetry/configuration-maintainers
cc @open-telemetry/cpp-maintainers

With this PR, every type:

• Present in the yaml schema in https://github.com/open-telemetry/opentelemetry-configuration,
• Marked as stable in the schema (*)

is:

• Implemented in the yaml parser in opentelemetry-cpp
• Documented in the C++ language support status.

Feature coverage for all stable types is now complete for C++.

(*) Type SpanKind is not implemented, but it is only reachable using ExperimentalComposableRuleBasedSamplerRule, so it will not be found in a yaml config file using stable constructs only.