chore: gitignore credentials for docs

Adds standard credential gitignore (.env / *.pem / .secrets/ / .auth_token).
Per-CEO directive 2026-04-16: every plugin and template repo should
gitignore credentials so self-hosters can't accidentally commit real
tokens to public repos.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitignore

@@ -42,3 +42,17 @@ next-env.d.ts
 # IDE
 .vscode/
 .idea/
+
+# Credentials — added by chore/credentials-gitignore batch
+.env.local
+.env.*.local
+.env.*
+!.env.example
+!.env.sample
+*.key
+*.crt
+*.p12
+*.pfx
+.secrets/
+.auth-token
+.auth_token