Add documentation for OTLP File Exporter.

[lalitb]

          > > Nice work. Can we document this exporter as an section in [README.md](https://github.com/open-telemetry/opentelemetry-cpp/tree/main/exporters/otlp) ?

Sorry for the missing. I will add documents tomorrow several days later. Sorry, I'm too busy recently.I will commit and push documents together with codes if there are more feedbacks about this PR.

Merging this PR now, please open a PR later for some doc when you have time.

Thanks for this work.

Originally posted by @marcalff in #2540 (comment)