Add readme files describing model execution flow for ASR tasks
Signed-off-by: smajumdar <titu1994@gmail.com>

# What does this PR do ?

Adds high level architecture / model execution flow diagrams to better enable users to understand how to write model scratch training / fine-tuning steps / inference steps

**Collection**: [ASR]

# Changelog 
- Add flow diagrams for CTC/RNNT model training
- Add flow diagram for ASR inference
- Add flow diagram for Speech classification
- Add flow diagram for Speech SSL

# Before your PR is "Ready for review"
**Pre checks**:
- [ ] 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?
- [ ] 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
