text: Title: GRADIO_SSR_MODE is not documented
  Body: ### Describe the bug
  
  The environment variable `GRADIO_SSR_MODE` is **not** documented in https://www.gradio.app/guides/environment-variables but referenced on these two lines: https://github.com/gradio-app/gradio/blob/3a6151e9422cc3b02b6e815cb5c72f59bebc60c7/gradio/blocks.py#L2460 and https://github.com/gradio-app/gradio/blob/3a6151e9422cc3b02b6e815cb5c72f59bebc60c7/gradio/routes.py#L1621
  
  ### Have you searched existing issues?  🔎
  
  - [X] I have searched and found no existing issues
  
  ### Reproduction
  
  ```python
  import gradio as gr
  
  # This is not a code issue. The GRADIO_SSR_MODE will be picked up correctly by the Gradio server but it is not documented.
  ```
  
  
  ### Screenshot
  
  _No response_
  
  ### Logs
  
  _No response_
  
  ### System Info
  
  ```shell
  Gradio Environment Information:
  ------------------------------
  Operating System: Linux
  gradio version: 5.7.1
  gradio_client version: 1.5.0
  
  ------------------------------------------------
  gradio dependencies in your environment:
  
  aiofiles: 23.2.1
  anyio: 4.7.0
  audioop-lts is not installed.
  fastapi: 0.115.6
  ffmpy: 0.4.0
  gradio-client==1.5.0 is not installed.
  httpx: 0.28.0
  huggingface-hub: 0.26.3
  jinja2: 3.1.4
  markupsafe: 2.1.5
  numpy: 2.1.3
  orjson: 3.10.12
  packaging: 24.2
  pandas: 2.2.3
  pillow: 11.0.0
  pydantic: 2.10.3
  pydub: 0.25.1
  python-multipart==0.0.12 is not installed.
  pyyaml: 6.0.2
  ruff: 0.8.2
  safehttpx: 0.1.6
  semantic-version: 2.10.0
  starlette: 0.41.3
  tomlkit==0.12.0 is not installed.
  typer: 0.15.1
  typing-extensions: 4.12.2
  urllib3: 2.2.3
  uvicorn: 0.32.1
  authlib; extra == 'oauth' is not installed.
  itsdangerous; extra == 'oauth' is not installed.
  
  
  gradio_client dependencies in your environment:
  
  fsspec: 2024.9.0
  httpx: 0.28.0
  huggingface-hub: 0.26.3
  packaging: 24.2
  typing-extensions: 4.12.2
  websockets: 12.0
  ```
  
  
  ### Severity
  
  I can work around it