Docs: broken internal links

## Describe the bug

Internal links in the docs hosted on https://chainsafe.github.io/gossamer appear broken.

## Expected Behavior

Click on a link and navigates to relevant page.

## Current Behavior

- Internal links are broken with a 404

## Possible Solution

Use mkdocs.yml `site_url: https://chainsafe.github.io/gossamer`

## To Reproduce
Steps to reproduce the behaviour:

1. Browse to https://chainsafe.github.io/gossamer/
2. Click on an internal link in the docs body e.g. "Get Started" or "General Resources"
- Observe it navigates to https://chainsafe.github.io/getting-started/resources/general-resources/ (404) rather than https://chainsafe.github.io/gossamer/getting-started/resources/general-resources/

- Version: current docs chainsafe.github.io (As of 2021-12-17)