ci: rename deploy secret DOCS_DEPLOY_KEY -> WEBSITE_DEPLOY_KEY

start-os, start-docs, and start9.com all deploy to the same VPS with the
same key; standardize the secret name across the three website repos so
it's obvious in GitHub that it's one shared key.

Note: the WEBSITE_DEPLOY_KEY secret (same value as the current
DOCS_DEPLOY_KEY) must exist in the repo before this merges, or the next
deploy will fail on an empty key.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: MattDHill

.github/workflows/deploy.yml

@@ -46,7 +46,7 @@ jobs:
       - name: Configure SSH
         run: |
           mkdir -p ~/.ssh
-          echo "${{ secrets.DOCS_DEPLOY_KEY }}" > ~/.ssh/deploy_key
+          echo "${{ secrets.WEBSITE_DEPLOY_KEY }}" > ~/.ssh/deploy_key
           chmod 600 ~/.ssh/deploy_key
           ssh-keyscan -H start9.com >> ~/.ssh/known_hosts
           cat >> ~/.ssh/config <<EOF