docs: replace vuepress with vitepress

[haoqunjiang]

Vuepress didn't work well with the project at the moment, maybe due to the conflicting webpack versions. The Netlify deployment was failing.

Notable changes after switching to Vitepress:

1. The default theme doesn't support nested nav links, so I have to revert docs: merge plugin-related navigation items #5655 to use a single-level dropdown for "plugins".
2. The carbon ad now displays at bottom-right instead of top-left.
3. No i18n for vite-plugin-pwa prompts.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Underlying tools
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

Other information: