New documentation site and some other changes

[CaiJimmy]

The old documentation site has not been updated for a long time, especially the Chinese version of it, which causes much confusion due to the outdated information.

Since there's not enough time to maintain both versions, I've taken the decision to keep only the English version of it. The Chinese version is no longer available on the new site, but it's still available on the old site

I've moved the tool used to generate the documentation site from VuePress to VitePress. It's much faster than VuePress and has a nicer default theme.

Some other changes:

• Documentation site is now hosted on Vercel, and the URL is changed to https://stack.jimmycai.com/
• Demo site is now pointed to https://github.com/CaiJimmy/hugo-theme-stack-starter
• The old demo site is accessible at https://hugo-theme-stack.netlify.app/. This should only be used for development purposes (testing features).

I hope the new documentation will be more helpful and easier to use. If you find any issues, please let me know.