Make repo OSS ready

[pimvernooij]

We want to open source this repo soon, and I think we should do a couple things for that. Perhaps not all blocking for release.

• [x] Add license (MPL-2.0)
• [x] Release pipeline via github actions (building, testing & pushing to terraform registry)
• [x] Proper readme
• [x] Changelog
• [x] Docs folder with mkdocs (that eventually pushes to readthedocs via an action?)
• [ ] Code coverage

After review another addition:

• [x] Amplience credentials needs to come from provider configuration instead of environment variables
• [x] Rename content_api_path to api_url
• [x] Store webhooks headers in state in order to be able to compare
• [x] Store webhooks notifications in state in order to be able to compare
• [x] Avoid force re-create when headers are used. Describe steps when a 'secret' value is changed. The diff won't pick this up because it's not part of the state file. We need to describe steps in order to update the webhook (by running terraform taint on that resource for example)

Nice to have / low-prio

• [x] Mask 'secret' header values. We could maybe do this by having separate 'value' and 'secret_value';

  "value": {
      Type:     schema.TypeString,
      Optional: true,
      ConflictsWith: []string{"secret_value"}
  },
  "secret_value": {
      Type:     schema.TypeString,
      Optional: true,
      Sensitive: true,
      ConflictsWith: []string{"value"}
  },

• [x] Allow overwrite of auth_url
• [ ] Expand test to verify that the terraform update actually performs an update instead of a re-create