SEO improvement

[lizable]

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, 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. Maybe we can insert meta tags in each section of the menu. Any other ideas are welcome, too.

[jung-min-seo]

https://developers.google.com/search/docs/appearance/visual-elements-gallery?hl=ko#attribution
https://developers.google.com/search/docs/appearance/site-names?hl=ko

I think we need to give each page a new name, such as SiteName to indicate that it's a page about installing the WebUI, not just Backend.ai, while keeping the application/ld+json stuff in.

It would be nice to be able to output the subdomain address as SiteName.