Improve documentation of review process

[acka47]

After some time without working on SkoHub, both @Phu2 and me were confused about the process, see #327 (comment)

We should add something like this to the documentation, maybe at https://github.com/skohub-io/skohub-vocabs/blob/main/CONTRIBUTING.md#functional-review:

The usual approach:

1. We merge feature branches into the dev branch.

2. This triggers a rebuild of test.skohub.io based on the dev branch.

3. Usually, I am the one to test new features. If needed I adjust/add vocabs in https://github.com/acka47/testing-skohub-vocabs and then trigger a build on test.skohub.io to do the functional review.

4. If needed, adustments are made in the feature branch and back to 1.).

5. If I approve the changes, the feature branch is merged into main.

It might make sense to discuss this in the next development meeting.

[acka47]

As discussed in today's meeting, I will take a look how to integrate this with l minimal changes into the CONTRIBUTING.md.

[acka47]

I think it makes a lot of sense to set up a repo skohub-io/testing-skohub-vocabs with a webbhook configured for deploying changes on test.skohub.io. With this, anybody who has write access to the skohub-io organization will be able to trigger builds on test.skohub.io for functional review.