Add OpenGraph metadata for better links to NeMo Website
# What does this PR do ?

Adds [OpenGraph](https://ogp.me/) support to NeMo website for richer URL linking.

**Collection**: [Docs]

# Usage

```markdown

---
title: Template 2022
author: [First Name, Second Name]
author_gh_user: [Github User 1, Github User 2]
read_time: Time to read
publish_date: Date of publishing

# Optional: OpenGraph metadata
og_title: Title of the blog post for Rich URL previews
og_image (or image): Image for Rich URL previews (absolute URL)
og_image_type: Image type (e.g. image/png). Defaults to image/png.
page_path: Relative path to the image from the website root (e.g. /assets/images/)
description: Description of the post for Rich URL previews

# DO NOT CHANGE _BELOW
template: blog.html
---
```

# 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**:
- [x] New Feature
- [ ] Bugfix
- [x] Documentation
