Allow Group Descriptions to Appear in Generated dbt Docs

### Describe the feature
Support for rendering group-level descriptions in dbt docs.

Sample:

finance.yml
```
groups:
- name: finance
owner:
# 'name' or 'email' is required; additional properties will no longer be allowed in a future release
email: finance@jaffleshop.com
description: For the finance team
config:
meta: # optional
data_owner: Finance team
```

The description provided under a group (e.g., "For the finance team") should appear in the generated dbt docs

### Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

### Additional context
Is this feature database-specific? Which database(s) is/are relevant? Please include any other relevant context here.

### Who will this benefit?
What kind of use case will this feature be useful for? Please be specific and provide examples, this will help us prioritize properly.

### Are you interested in contributing this feature?
Let us know if you want to write some code, and how we can help.