docs(tools): Added a note about PIP_INDEX_URL for better mirrors in some regions

Closes https://github.com/espressif/esp-idf/issues/18301

docs/en/get-started/linux-macos-setup-legacy.rst

@@ -211,11 +211,13 @@ To prefer the Espressif download server when installing tools, use the following
 
 .. note::
     For users in China, we recommend using our download server located in China for faster download speed.
+    Alternatively, you can set as well the environment variable ``PIP_INDEX_URL`` to the China mirror of PyPI.
 
     .. code-block:: bash
 
         cd ~/esp/esp-idf
         export IDF_GITHUB_ASSETS="dl.espressif.cn/github_assets"
+        export PIP_INDEX_URL="https://mirrors.tuna.tsinghua.edu.cn/pypi/web/simple"
         ./install.sh
 
 Customizing the Tools Installation Path

docs/zh_CN/get-started/linux-macos-setup-legacy.rst

@@ -211,11 +211,13 @@ ESP-IDF 工具安装器会下载 Github 发布版本中附带的一些工具，
 
 .. note::
     推荐国内用户使用国内的下载服务器，以加快下载速度。
+    也可以同时设置环境变量 ``PIP_INDEX_URL`` 为国内的 PyPI 镜像，以加快 Python 包的下载速度。
 
     .. code-block:: bash
 
         cd ~/esp/esp-idf
         export IDF_GITHUB_ASSETS="dl.espressif.cn/github_assets"
+        export PIP_INDEX_URL="https://mirrors.tuna.tsinghua.edu.cn/pypi/web/simple"
         ./install.sh
 
 自定义工具安装路径