[Docs] - Add Getting Started docs, and Reference API document

[lalitb]

• Getting started docs can be added here - https://github.com/open-telemetry/opentelemetry.io/tree/main/content/en/docs/instrumentation/cpp
• Reference API for trace and metrics is here - https://labhas-opentelemetry-cpp.readthedocs.io/en/latest/otel_docs/otel_docs.html. We can add for logs too. Refer this PR on how the metrics was integrated - Fix:#1575 API Documentation for Metrics SDK and API #1678

[github-actions]

This issue was marked as stale due to lack of activity.