* Upgrade from Spinx 3.x to **Sphinx 4.x**.
* Added features:
* Add headbar background color theme option.
* Add local table of contents support in the sidebar.
* Add a social icon for Bilibili.
* Add **video embedding** support for ``<iframe>`` HTML videos.
* Add **subfolder** support. Now users can switch languages on subfolder docs, but the headbar support for subpages still need improved.
* Error fix:
* Fixed the problem that the social link feature always points to empty URL.
* Improve the organization and size of the installation package.