Update NeMo github.io website
# What does this PR do ?

Modernize and update NeMo Github.io website.

**Collection**: [Docs]

# Changelog 
- Replace previous redirect with mkdocs for material based website

# Usage
- To build the website locally to see the contents, run `bash build_docs.sh` from the `gh-pages` branch.
- In order to add posts, follow the steps in the template inside `docs/blogs/template.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?
- [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**:
- [ ] New Feature
- [ ] Bugfix
- [x] Documentation

