Move CONTRIBUTING.MD content to the website/documentation

[magdmartin]

Following the last round of edits in GOVERNANCE.MD OpenRefine/OpenRefine#6737 @wetneb proposed to:

1. improve the documentation on the website with whatever information is in CONTRIBUTING.md and not there yet
2. change CONTRIBUTING.md to be a redirection to the website as suggested below

Thank you for your interest in contributing to OpenRefine! Our documentation for new contributors is at:
https://openrefine.org/docs/technical-reference/contributing

Originally posted by @wetneb in OpenRefine/OpenRefine#6737 (comment) - edited by @magdmartin

[aakash-a-dev]

It makes sense, been quite long but I am getting started with this.