docs-builder: add pull-requests: write permission to docs-build workflow (elastic#224714)

## Details

⚠️ This PR was created by an automated tool. Please review the changes
carefully. ⚠️

Add permissions `pull-requests: write` to the docs-build workflow.

## Why

We want to comment on the PR with the links to modified docs pages.

See elastic/docs-builder#1395 for further
details

Author: reakaleek

.github/workflows/docs-build.yml

@@ -19,4 +19,4 @@ jobs:
       deployments: write
       id-token: write
       contents: read
-      pull-requests: read
+      pull-requests: write