Add docs validation workflow (#1994)

* Add docs validation workflow

* Update ref

* Unpin ref

Author: Gleb

.github/workflows/docs.yml

@@ -0,0 +1,16 @@
+name: Docs validation
+
+on:
+  push:
+    branches: [main, release/**]
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref_protected == 'true' && github.sha || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  docs-validation:
+    uses: stellar/stellar-docs/.github/workflows/build.yml@main
+    with:
+      stellar-cli-ref: ${{ github.ref }}