Add documentation for version 7.1

[pingyu]

What is changed?

• Add docs for LTS version 7.1:
  • Overall, version list on the top: https://deploy-preview-349--tikv.netlify.app/
  • Release notes for 7.1: https://deploy-preview-349--tikv.netlify.app/docs/7.1/new-features/overview/
  • Release notes for dev: https://deploy-preview-349--tikv.netlify.app/docs/dev/new-features/overview/ (currently nothing)
• Add guide about how a add a new release Preview: https://github.com/pingyu/tikv-website/blob/add-version-7.1/CONTRIBUTING.md#add-a-version

Any related PRs or issues?

Which version does your change affect?

v7.1, dev

[iosmanthus]

Pretty cool! Are the release-7.1 docs ported from the older version?

[pingyu]

Pretty cool! Are the release-7.1 docs ported from the older version?

Mostly copy from dev, except for release notes.

[pingyu]

cc @zeminzhou PTAL~