[C++] Manual Instrumentation Page

[svrnm]

via open-telemetry/opentelemetry-cpp#1589

We'd like to grow the user docs for C++ on opentelemetry.io, for that we need a "Manual Instrumentation" page, similar to what we already have for other languages. In a first version it should talk about:

• Acquiring a tracer
• Create Spans
• Create nested spans
• get the current span
• span attributes + semantic attributes

More topics can follow in a second version

For reference (and to copy some text), some other languages:

• https://opentelemetry.io/docs/instrumentation/js/instrumentation/
• https://opentelemetry.io/docs/instrumentation/java/manual/
• https://opentelemetry.io/docs/instrumentation/net/manual/