SEO improvement

> NOTE: @jung-min-seo issued this for better search results on search engines. Thanks a lot!

We have been providing our documentation as a site(https://webui.docs.backend.ai) by using [readthedocs](https://docs.readthedocs.io/en/stable/index.html), and it definitely helps those who want to get information about Backend.AI and other products, I think it's crucial to improve SEO of our documentation.

I found that adding proper meta tags is helpful to improve the SEO of the site according to the [technical-docs-seo-guide in readthedocs guide](https://docs.readthedocs.io/en/stable/guides/technical-docs-seo-guide.html). Maybe we can insert meta tags in each section of the menu. Any other ideas are welcome, too.