[Doc] Update index and TOC tree (open-mmlab#767)

* stash

* cleanup

* use relpath for dataset.md and modelzoo.md

* basename for reference

* add brief introduction in index

* add remove notification

* typo

* minor update

* recover language switcher

* missing switch language caption

* minor

* Update switch_language.md

* revert

docs/en/index.rst

@@ -1,7 +1,27 @@
 Welcome to MMEditing's documentation!
 =====================================
 
-You can switch between Chinese and English documents in the bottom-left corner of the layout.
+Languages:
+`English <https://mmediting.readthedocs.io/en/latest/>`_
+|
+`简体中文 <https://mmediting.readthedocs.io/zh_CN/latest/>`_
+
+MMEditing is an open-source toolbox for low-level vision.
+It supports various tasks, including:
+
+* Image super-resolution
+* Video super-resolution
+* Video frame interpolation
+* Image inpainting
+* Image matting
+* Image generation
+* and possibly more in the future.
+
+It is based on `PyTorch <https://pytorch.org>`_ and is a part of the `OpenMMLab project <https://openmmlab.com/>`_.
+Codes are available on `GitHub <https://github.com/open-mmlab/mmediting>`_.
+
+Documentation
+=============
 
 .. toctree::
    :maxdepth: 2
@@ -52,21 +72,17 @@ You can switch between Chinese and English documents in the bottom-left corner o
 
 .. toctree::
    :maxdepth: 2
-   :caption: Notes
+   :caption: API Reference
 
-   notes.md
+   api.rst
 
 .. toctree::
    :maxdepth: 2
-   :caption: FAQ
+   :caption: Miscellaneous
 
    faq.md
-
-.. toctree::
-   :maxdepth: 2
-   :caption: API Reference
-
-   api.rst
+   changelog.md
+   notes.md
 
 .. toctree::
    :caption: Switch Language