docs(mirror): Add Chinese mirror links

[lucasssvaz]

Description of Change

This pull request updates the documentation to provide alternative download links for users in China, addressing potential issues with slow or unstable connections to GitHub. The changes include adding references to the Jihulab mirror in the installation and troubleshooting guides.

Documentation updates for users in China:

• docs/en/installing.rst: Added a note and alternative repository link (https://jihulab.com/esp-mirror/espressif/arduino-esp32.git) for users in China experiencing slow download speeds. Also included stable and development release JSON file links for the Arduino IDE Boards Manager. [1] [2]

• docs/en/troubleshooting.rst: Added a new section titled "Slow or unstable downloads" with Jihulab mirror links for the repository and JSON files, providing guidance for users facing connection issues.

Requires #11288 to be consistent.

[github-actions]

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[pedrominatel]

LGTM.