Use sticky sidebar for docs site

[bobaekang]

Simple sticky sidebar can significantly improve the UX for navigating the docs site. See the following comparisons:

Large Screen

Current

https://user-images.githubusercontent.com/22449454/169185059-138e9552-8ae7-4323-a637-acd1f46ed63e.mp4

With sticky sidebar

https://user-images.githubusercontent.com/22449454/169185085-c67a4f36-fc8d-49f3-8b27-3269a0fb673f.mp4

Small Screen

Current

https://user-images.githubusercontent.com/22449454/169185043-fbf1dd9e-5941-4126-ae89-58e4ea96808f.mp4

With sticky sidebar

https://user-images.githubusercontent.com/22449454/169185023-6c2d98d7-0ab5-4740-a581-444836211084.mp4