Fix documentation for Numba
# What does this PR do ?

Fix import of numba to prevent doc breakage.  

importlib cannot actually import a module that is mocked, trying to call `module.__version__` will return a mock module object - not a string and then Version() will fail to parse that mock object.

This PR defers the attempt to find the version compatibility of FP16 to the moment the library is resolved dynamically rather than at static import time.

**Collection**: [Core]

# Changelog 
- Correct numba import to be dynamic only to avoid mock import failing
- Fix NeMo version in docs and readme.md

# 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?
- [ ] Did you add or update any necessary documentation?
- [ ] 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**:
- [ ] New Feature
- [x] Bugfix
- [ ] Documentation
