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 Drops outdated runner content from the FAQ and design pages.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-cn/docusaurus-plugin-content-docs/current/usage/actions/scoped-workflows.md

@@ -102,8 +102,8 @@ SCOPED_WORKFLOW_DIRS = .gitea/scoped_workflows
 
 作用域工作流可以调用可复用工作流：
 
-- **本地**引用（`uses: ./...`）会相对于**源**仓库解析：即调用方工作流内容的来源仓库，而非消费方仓库。
-- **跨仓库**引用（`uses: owner/repo/...@ref`）会以**消费方仓库的**读取权限来解析。如果它指向一个私有仓库，请确保每个消费方仓库都能读取它，否则工作流会在那里失败。
+- **本地**引用（`uses: ./...` 或 `uses: $/...`）会相对于**源**仓库解析：即调用方工作流内容的来源仓库，而非消费方仓库。
+- **跨仓库**引用（`uses: owner/repo/...@ref` 或 `uses: self:owner/repo/...@ref`）会以**消费方仓库的**读取权限来解析。如果它指向一个私有仓库，请确保每个消费方仓库都能读取它，否则工作流会在那里失败。
 
 可复用工作流可以放在源仓库的 `SCOPED_WORKFLOW_DIRS`（或 `WORKFLOW_DIRS`）之下。