doc/admin: refactor OpenTelemetry documentation

Created by: bobheadxi

Best reviewed as a whole. Key ideas with the rewrites here:

1. Treat OpenTelemetry exporters as a standalone topic - omit mention of exporters you can't use yet (such as Loki)
2. Focus on tracing in the Tracing section, linking out to the exporters section as needed.
3. Condense intro section, add more mentions that this is WIP and we only support tracing right now
4. Improve link-ability of headings by making them shorter

Part of https://github.com/sourcegraph/sourcegraph/issues/40680

Test plan

sg run docsite -> http://localhost:5080/admin/observability/opentelemetry