[C++] Resources 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 "Resources", with the following content (if applicable):

• Resources that are detected out of the box
• Adding resources via OTEL_RESOURCE_ATTRIBUTES
• Adding resources via code
• Resource Detector example (e.g. container id)

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

For reference (and to copy text), the JS page on the same topic: https://opentelemetry.io/docs/instrumentation/js/resources/