docs: add hermes postinstall to installation + quickstart, fix update --check description

- installation.md: add tip about `hermes postinstall` for upfront dep install
- quickstart.md: show `hermes postinstall` in pip install flow
- updating.md: fix --check description to mention PyPI path for pip installs

website/docs/getting-started/quickstart.md

@@ -52,6 +52,7 @@ Pick the row that matches your goal:
 
 ```bash
 pip install hermes-agent
+hermes postinstall     # optional: installs Node.js, browser, ripgrep, ffmpeg + runs setup
 ```
 
 PyPI releases track tagged versions (major/minor releases), not every commit on `main`. For bleeding-edge, use Option B.