ci: add "Pull Request Labeler" GitHub Action

introduces the concept of a PR labeler to llama-stack
github repo. only documentation PRs implemented in
this commit.

Signed-off-by: Nathan Weinberg <nweinber@redhat.com>

.github/labeler.yml

@@ -0,0 +1,4 @@
+documentation:
+  - changed-files:
+    - any-glob-to-any-file:
+      - "*.md"

.github/workflows/label.yml

@@ -0,0 +1,14 @@
+name: Pull Request Labeler
+
+on:
+  pull_request_target:
+
+jobs:
+  labeler:
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+      pull-requests: write
+    steps:
+    - name: Labeler Action
+      uses: actions/labeler@v5