Move CONTRIBUTING.MD content to the website/documentation

[magdmartin]

Following the last round of edits in GOVERNANCE.MD https://github.com/OpenRefine/OpenRefine/pull/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 https://github.com/OpenRefine/OpenRefine/issues/6737#issuecomment-2282706320 - edited by @magdmartin