release v1.9 docs and remove v1.6 docs

[chivalryq]

Signed-off-by: Jianbo Sun jianbo.sjb@alibaba-inc.com### Description of your changes

I have:

• [x] Read and followed KubeVela's contribution process.
• [ ] Update sidebar.js if adding a new page.
• [x] Run yarn start to ensure the changes has taken effect.

Special notes for your reviewer