[DOC] Migrate CONTRIBUTING.md to website

[agardnerIT]

Change in the documentation

Move https://github.com/open-feature/flagd/blob/main/CONTRIBUTING.md to the website

[agardnerIT]

Closed the above PR, this will be closed by #825

[beeme1mr]

The contributing doc should remain in the repo. It has special meaning in GitHub and would be where contributors would look.