docs(en): add readthedocs

docs/en/doc_add_readthedocs.md

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+# Adding `chat_with_repo` to readthedocs for Free
+
+This article introduces how to implement `chat_with_repo` on Read the Docs at zero cost. See [HuixiangDou Read the Docs documentation](https://huixiangdou.readthedocs.io).
+
+The deployment workflow is as follows:
+
+<img src="https://github.com/user-attachments/assets/d15935fa-a8fa-49ed-9995-7549ab1f71dc"  width="400">
+
+Where:
+* [Read the Docs](https://readthedocs.io) hosts Chinese and English documents
+* [OpenXLab](https://openxlab.org.cn/apps) provides an HTTPS entrance (Read the Docs cannot embed HTTP) and CPU
+* [SiliconCloud](https://siliconflow.cn/siliconcloud) provides text2vec, reranker, and LLM model APIs
+
+We need to use readthedocs custom theme, adding a button to the theme.
+
+1. When the button is clicked, create an `iframe` to load the HTTPS version of Huixiang Dou's Gradio UI
+2. HTTPS requires domain name verification. You can use the random subdomain provided by OpenXLab
+3. The GPU resources in OpenXLab are limited, so we use the free model APIs provided by SiliconCloud
+
+Here are the steps to follow.
+
+## I. Prepare Code and Documentation
+
+Assuming you use all the documents of `mmpose` as the knowledge base, put the knowledge base into repodir
+
+```bash
+cd HuixiangDou
+mkdir repodir
+git clone https://github.com/open-mmlab/mmpose  --depth=1
+# Remove the .git of the knowledge base
+rm -rf .git
+```
+
+Adjust the default configuration of `gradio_ui.py` to use `config-cpu.ini`
+```bash
+# huixiangdou/gradio_ui.py
+    parser.add_argument(
+        '--config_path',
+        default='config-cpu.ini',
+        type=str,
+..
+```
+
+Commit the knowledge base and the Huixiangou project to Github together, for example, the `for-openxlab-readthedocs` branch of [huixiangdou-readthedocs](https://github.com/tpoisonooo/huixiangdou-readthedocs/tree/for-openxlab-readthedocs).
+
+## II. Create an OpenXLab Application
+
+Open [OpenXLab](https://openxlab.org.cn/apps) and create a `Gradio` type application.
+
+1. Enter the Github address and branch name from the previous step
+2. Choose the CPU server
+
+After confirmation, modify the application settings:
+
+* Add `huixiangdou/gradio_ui.py` to `Customize Startup File` 
+* Since the code is open source, you need to configure environment variables. HuixiangDou prioritizes using the token in the `config-cpu.ini`. If it cannot be found, it will try to check `SILICONCLOUD_TOKEN` and `LLM_API_TOKEN`, as shown in the figure:
+
+    <img src="https://github.com/user-attachments/assets/66291c65-1a5e-495a-aad6-e8962bef6bb6"  width="400">
+
+Start the application. The first run takes about **10 minutes** to build the feature library. After it's done, you should see a gradio application. For example:
+
+```bash
+https://openxlab.org.cn/apps/detail/tpoisonooo/HuixiangDou-readthedocs 
+```
+
+Press F12 in the browser to check the source code and obtain the corresponding HTTPS address for this service:
+
+```JavaScript
+src="https://g-app-center-000704-0786-wrbqzpv.openxlab.space" 
+```
+
+As long as the application data is not deleted, this address is **fixed**.
+
+## III. Use Read the Docs Custom Theme
+
+Assuming you are already familiar with the basic usage of Read the Docs, you can directly copy the HuixiangDou docs directory
+
+* zh or en directory
+* requirements/doc.txt sets the custom theme
+
+[Here](https://github.com/tpoisonooo/pytorch_sphinx_theme/) is the implementation of our custom theme, mainly:
+
+1. Create a `chatButton` and an empty container in [layout.html](https://github.com/tpoisonooo/pytorch_sphinx_theme/blob/3db120b0f1e064425f37e98368dcea49972702e9/pytorch_sphinx_theme/layout.html#L324)
+2. Bind an event to `chatButton`. When the button is clicked, the empty container loads the HTTPS address, for example, the previous one:
+
+    ```bash
+    https://g-app-center-000704-0786-wrbqzpv.openxlab.space 
+    ```
+
+    You can modify your preferred style in [theme.css](https://github.com/tpoisonooo/pytorch_sphinx_theme/blob/master/pytorch_sphinx_theme/static/css/theme.css).
+
+Finally, configure your project on readthedocs.io, and `Build Version` will do.