[C++] Exporters Page

[svrnm]

via open-telemetry/opentelemetry-cpp#1589

We'd like to grow the user docs for C++ on opentelemetry.io. We want to have a page on "Exporters", that outlines how one can setup&configure an OTLP exporter.

In a later version we can also include other exporters if applicable.

Some other languages to copy from:

• https://opentelemetry.io/docs/instrumentation/net/exporters/
• https://opentelemetry.io/docs/instrumentation/js/exporters/