docs: document self:, builtin:, $/ and runner tracing (#553)

Documents the `self:` and `builtin:` action prefixes from https://github.com/go-gitea/gitea/pull/39358, https://gitea.com/gitea/runner/pulls/1233 and https://gitea.com/gitea/runner/pulls/871, workflow-relative `$/` actions, and the runner's OpenTelemetry tracing from https://gitea.com/gitea/runner/pulls/1206.

1. Trims the action resolution and `DEFAULT_ACTIONS_URL` docs to state each fact once.
1. Drops outdated runner content from the FAQ and design pages.
1. Syncs zh-cn and zh-tw, including a port of the token permissions page they now link to.

_Written by Claude._

Reviewed-on: https://gitea.com/gitea/docs/pulls/553
Reviewed-by: bircni <[email protected]>
Co-authored-by: silverwind <[email protected]>

runner-docs/monitoring.md

@@ -40,6 +40,16 @@ All series are prefixed with `gitea_runner_`, among them:
 
 Useful things to alert on: `gitea_runner_poll_fetch_total{result="error"}` rising (the instance is unreachable or the runner was deleted), a `capacity_utilization_ratio` pinned at `1` (the pool is too small), and a growing `report_log_buffer_rows` (log delivery is falling behind).
 
+## OpenTelemetry tracing
+
+Setting `OTEL_EXPORTER_OTLP_ENDPOINT` in the runner's environment sends a trace of every job, with a span per step, to an OTLP collector over `http/protobuf`:
+
+```sh
+OTEL_EXPORTER_OTLP_ENDPOINT=http://localhost:4318
+```
+
+Jobs receive `TRACEPARENT`, so instrumented tools in them join the job's trace. See [docs/telemetry.md](https://gitea.com/gitea/runner/src/branch/main/docs/telemetry.md) for headers, sampling and the security notes.
+
 ## Local health checks
 
 Health checks let a runner take itself out of rotation when the machine it runs on is not fit for work — most commonly when the disk is full.