[ZEPPELIN-6290] Add .env file support and update merge script documentation

### What is this PR for?

Add .env file support and update merge script documentation
- Added .env file to .gitignore to prevent dotenv files from being tracked.
- Updated `merge_zeppelin_pr.py` to load environment variables from a .env file, enhancing configuration management.
- Added `/dev/requirements.txt` to pin Python package versions and use it for package installation.
- Revised error message for missing JIRA library to provide clearer installation instructions.

### What type of PR is it?
Improvement

### What is the Jira issue?
[ZEPPELIN-6290](https://issues.apache.org/jira/browse/ZEPPELIN-6290)

### How should this be tested?
* Run `uv pip install -r {REPO_ROOT}/requirements.txt`
* Environment variable configurations could be tested by adding `{REPO_ROOT}/dev/.env` file and run `merge_zeppelin_pr.py`

### Questions:
* Does the license files need to update? No
* Is there breaking changes for older versions? No
* Does this needs documentation? No


Closes #5041 from tbonelee/add-dotenv.

Signed-off-by: ParkGyeongTae <gyeongtae@apache.org>