Add DLLogger support to exp_manager
# What does this PR do ?

Adds an option to log with DLLogger in the experiment manager.

**Collection**: [Note which collection this PR will affect]

# Changelog 
- Changed exp_manager.py to add a new logger
- Updated the exp_manager documentation

# Usage
In the exp_manager config:

```yaml
enable_dllogger_logger: True
dllogger_logger_kwargs:
    stdout: False
    verbose: False
    json_file: "./dllogger.json"
```

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [ ] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
- [x] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [ ] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to # (issue)
