doc/admin: clean up tracing documentation

Created by: bobheadxi

Restructures the docs (right now it's all over the place), removes datadog (functionality has been removed), adds a mention of OpenTelemetry (experimental). Also see https://github.com/sourcegraph/sourcegraph/pull/40442 which outlines the goal here (move towards "Jaeger is just a backend, and with 4.0 you can still have Jaeger, just via the opentelemetry collector)

Test plan

sg run docsite