Improve SEO for new website

Please do not create a Pull Request without creating an issue first.

## What is the purpose of the change
Now we have a new website, but its links don't work with the old website. So old site links on search engines will cause 404. We can google for site:rocketmq.apache.org to verify it, only the home page is accessible in the top results.

I propose to maintain compatibility with old site links to optimize SEO. There are two ways to do this:

1. make the new website links as same as the old one
2. set 301 redirects to route traffic from an old path to a new URL

Moreover, URLs on the new website look weird. Maybe better to use slug.

```
https://rocketmq.apache.org/docs/%E4%BB%8B%E7%BB%8D/03whatis
-> https://rocketmq.apache.org/docs/what-is-rocketmq
```

Start by auditing the new website's routing and comparing its URLs with links found through site:rocketmq.apache.org. Check how the old path /docs/%E4%BB%8B%E7%BB%8D/03whatis should map to /docs/what-is-rocketmq. Done means old search-result paths no longer produce 404 responses and the new URLs use agreed slugs.

Written by the indexing model from the issue text.