docs: update docs including OTEL and other integrations

[lcian]

Adds docs for OTEL, basically what's already in sentry-opentelemetry/src/lib.rs.

Also mentions the actix and tracing integrations in the main README.md.

[Swatinem]

you do not strictly need to update these manually. the release workflow is using cargo-readme to update it based on the toplevel crate docs.

[lcian]

you do not strictly need to update these manually. the release workflow is using cargo-readme to update it based on the toplevel crate docs.

Ah that's great, I didn't know that :).