Use Gitlab for OIDC authentication

[!IMPORTANT]
Documentation Update: Product documentation, which were located in this repository under /website, are now located in hashicorp/web-unified-docs, colocated with all other product documentation. Contributions to this content should be done in the web-unified-docs repo, and not this one. Changes made to /website content in this repo will not be reflected on the developer.hashicorp.com website.

Visit Settings > Applications.
Fill out Name and Redirect URIs.
Making sure to select the "openid" scope.
Copy client ID and secret.