Fix link to documentation on how to contribute (#3403)

The previous link was broken (404 error), so this is an improvement.
However, an even better fix would be to have a general documentation
landing page for all types of contributions, and it would refer out to
the type-specific sections (e.g., translations, code contributions, doc
contributions).

Currently, because there is no such page, we have to instead point to
https://docs.hypha.app/getting-started/contributing/code-contributions/
(i.e., https://docs.hypha.app/getting-started/contributing/ does not
exist -- that URL also gets a 404 error). I think a better fix here
would be to first make the latter exist in the documentation, and then
re-update the links in CONTRIBUTING.md here to point to the new place.
But let not the perfect be the enemy of the good. This fix is still a
great improvement, as it at least gets the user to contribution docs.