Login to third-party sites with Moera account

Third-party sites may place a DIV like <div class="moera-login" data-href="...">...</div>. If the Moera extension is installed, it will replace this code with a button Login with Moera. When user clicks the button, the extension redirects the browser to the home node with all parameters to perform login. The home node makes a request to the provided URL (that was in data-href) and gets a challenge in response. Then it adds a random value to the challenge, signs it and sends the random value it used and the signed challenge back. After verifying the signature, the target site returns the final URL and the home node redirects the browser to it.

Need to document this process in Moera overview and check if some standard protocol (like OAuth 2.0 or OpenID) may be used for it.

Start with the Moera overview and the login flow described in this issue. Document the extension, home-node, challenge, signature, and redirect steps, then compare the flow with OAuth 2.0 and OpenID. Done means the overview explains the process and records whether an existing standard protocol is suitable.