Add docs for bot PR triagebot handler

cc rust-lang/triagebot#1866

Author: Mark-Simulacrum

src/SUMMARY.md

@@ -38,6 +38,7 @@
     - [Triagebot Dashboard](./triagebot/triage-dashboard.md)
     - [Zulip Meeting Management](./triagebot/zulip-meeting.md)
     - [Zulip Notifications](./triagebot/zulip-notifications.md)
+    - [GitHub Actions created PR open/closer](./triagebot/bot-pull-requests.md)
 - [Community](./community/README.md)
     - [State of Rust Survey FAQ](./community/survey-faq.md)
 - [Compiler](./compiler/README.md)

src/triagebot/bot-pull-requests.md

@@ -0,0 +1,17 @@
+# GitHub Actions created PR open/closer
+
+This automation triggers an automatic close & reopen on PRs opened by the
+`github-actions` user, i.e., from a GitHub actions job. This enables CI to run
+on those PRs without needing a manual poke from some human.
+
+## Configuration
+
+This feature is enabled on a repository by having a `[bot-pull-requests]` table in `triagebot.toml`:
+
+```toml
+[bot-pull-requests]
+```
+
+## Implementation
+
+See [`src/handlers/bot_pull_requests.rs`](https://github.com/rust-lang/triagebot/blob/HEAD/src/handlers/bot_pull_requests.rs).