### Why are the changes needed?
The PR resolves multiple `"Title underline too short"` warnings to reduce noise during documentation building, for instance:
```shell
./kyuubi/docs/client/jdbc/mysql_jdbc.rst:18: WARNING: Title underline too short.
`MySQL Connectors`_
================ [docutils]
./kyuubi/docs/connector/hive/paimon.rst:17: WARNING: Title underline too short.
`Apache Paimon (Incubating)`_
========== [docutils]
./kyuubi/docs/connector/hive/paimon.rst:31: WARNING: Title underline too short.
Apache Paimon (Incubating) Integration
------------------- [docutils]
```
### How was this patch tested?
Checked that there are no `"Title underline too short"` warnings during the documentation build process.
```shell
make html
```
### Was this patch authored or co-authored using generative AI tooling?
No
Closes#6969 from dnskr/doc-fix-title-underline-too-short.
Closes#6969
2007a2440 [dnskr] [DOC] Fix "Title underline too short" issues
Authored-by: dnskr <dnskrv88@gmail.com>
Signed-off-by: Kent Yao <yao@apache.org>
085a297dee