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 Drops outdated runner content from the FAQ and design pages.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]>

i18n/zh-tw/docusaurus-plugin-content-docs/current/usage/actions/scoped-workflows.md

@@ -102,8 +102,8 @@ SCOPED_WORKFLOW_DIRS = .gitea/scoped_workflows
 
 作用域工作流程可以呼叫可重複使用的工作流程：
 
-- **本機**參照（`uses: ./...`）會相對於**來源**儲存庫解析：即呼叫方工作流程內容的來源儲存庫，而非消費方儲存庫。
-- **跨儲存庫**參照（`uses: owner/repo/...@ref`）會以**消費方儲存庫的**讀取權限來解析。如果它指向一個私有儲存庫，請確保每個消費方儲存庫都能讀取它，否則工作流程會在那裡失敗。
+- **本機**參照（`uses: ./...` 或 `uses: $/...`）會相對於**來源**儲存庫解析：即呼叫方工作流程內容的來源儲存庫，而非消費方儲存庫。
+- **跨儲存庫**參照（`uses: owner/repo/...@ref` 或 `uses: self:owner/repo/...@ref`）會以**消費方儲存庫的**讀取權限來解析。如果它指向一個私有儲存庫，請確保每個消費方儲存庫都能讀取它，否則工作流程會在那裡失敗。
 
 可重複使用的工作流程可以放在來源儲存庫的 `SCOPED_WORKFLOW_DIRS`（或 `WORKFLOW_DIRS`）之下。