Updates workflow file for docs build to use deploy token instead of legacy docs deploy key

[froschdesign]

Reference:

• https://github.com/laminas/documentation-theme/blob/master/github-actions/docs/README.md#environment
• https://docs.github.com/en/actions/security-guides/automatic-token-authentication#using-the-github_token-in-a-workflow

[Ocramius]

I don't have the knowledge to say OK or KO to this, but go ahead, merge and release on your own call, @froschdesign :+1: