ci: remove GitHub CI — native git.hanzo.ai + cd.hanzo.ai only

This workflow ran on a self-hosted actions-runner-controller pool. ARC is
decommissioned: CI executes inside git.hanzo.ai (Gitea Actions, .hanzo/workflows)
and delivery is cd.hanzo.ai reconciling the reviewed image pin in
hanzoai/universe. GitHub is a mirror and runs nothing.

Removed rather than repointed — there is no GitHub-side pool to move to, and
leaving it would keep ARC load-bearing.

.github/workflows/docs-checker.yml

@@ -1,25 +0,0 @@
-name: 'Documentation Required'
-on:
-  pull_request:
-    types: [ opened, synchronize, reopened, edited ]
-
-jobs:
-  build:
-    runs-on: hanzo-build-linux-amd64
-    timeout-minutes: 10
-
-    steps:
-      - name: 'Setup Environment'
-        uses: actions/setup-python@v2
-        with:
-          python-version: '3.10'
-
-      - name: 'Clone repo'
-        uses: actions/checkout@v2
-
-      - name: Validate Docs
-        env:
-          PR_NUMBER: ${{ github.event.number }}
-        run: |
-          pip install requests --quiet
-          python $GITHUB_WORKSPACE/.github/helper/documentation.py $PR_NUMBER