add: CI for GitHub pages

[MahdiBaghbani]

Based on Michiels implementation of docs.html we don't even need to build GitHub pages on spec.yaml changes since the script inside the page would fetch latest spec.

<script>
      const params = new URLSearchParams(window.location.search);
      const user = params.get('user');
      const repo = params.get('repo');
      const branch = params.get('branch');
      let changed = false;
      if (user === null) {
        params.set('user', 'cs3org');
        changed = true;
      }
      if (repo === null) {
        params.set('repo', 'OCM-API');
        changed = true;
      }
      if (branch === null) {
        params.set('branch', 'develop');
        changed = true;
      }
      if (changed) {
        params.sort();
        window.location = `?${params.toString()}`;
      }
      document.querySelector('redoc').setAttribute('spec-url', `https://raw.githubusercontent.com/${user}/${repo}/${branch}/spec.yaml`);
    </script>

[glpatcern]

OK, so the idea is to have the CI run on demand, but most of the time we won't even need to run it?

[MahdiBaghbani]

Yes, we just need to run it once and then play with URL query