### _Why are the changes needed?_
Build content of the connector document section
#### Main changes proposed in this PR
- the `integration` section is renamed to `connector` and lifted up
- connectors are separated by engines
- the usage guide is broken into 2 section: Admin Guide and User Guide
### _How was this patch tested?_
- [ ] Add some test cases that check the changes thoroughly including negative and positive cases if possible
- [x] Add screenshots for manual tests if appropriate
- [ ] [Run test](https://kyuubi.apache.org/docs/latest/develop_tools/testing.html#running-tests) locally before make a pull request
Closes#3061 from yaooqinn/3060.
Closes#3060
6932cbb8 [Kent Yao] [KYUUBI #3060] [Subtask][#3059] Build the content of connector document section
Authored-by: Kent Yao <yao@apache.org>
Signed-off-by: Kent Yao <yao@apache.org>
4864762342
