docs: upgrade vuepress to v2 (backport #1958)

[mergify]

This is an automatic backport of pull request #1958 done by Mergify.
Cherry-pick of ea32d99 has failed:

On branch mergify/bp/release/v5.1.x/pr-1958
Your branch is up to date with 'origin/release/v5.1.x'.

You are currently cherry-picking commit ea32d997.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   docs/.gitignore
	renamed:    docs/.vuepress/config.js -> docs/.vuepress/config.ts
	new file:   docs/.vuepress/styles/index.scss
	deleted:    docs/.vuepress/styles/index.styl
	new file:   docs/.vuepress/styles/palette.scss
	deleted:    docs/.vuepress/styles/palette.styl
	modified:   docs/scripts/pre.sh

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   docs/package.json
	both modified:   docs/yarn.lock

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/github/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

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