Add peering API diagram

We could add the diagram using mermaid syntax for markdown, like so:
sequenceDiagram
    participant Meta
    participant PeeringDB
    participant Peer
    Peer->>Meta: Request URL for login
    Meta->>Peer: Sends back URL
    Peer->>PeeringDB: Sends request for auth code
    PeeringDB->>Peer: Replies back with auth code
    Peer->>Meta: Sends auth code to Meta
    Meta->>PeeringDB: Exchange auth code for token
    PeeringDB->>Meta: Send token back to Meta
    Note left of Meta: Now Meta knows what peer can do
    Meta->>Peer: Send OK back to peer
    Peer->>Meta: QUERY peering locations (peer type, ASN, auth code)
    Meta->>Peer: Meta replies back<br/>200 (peering locations, request ID)<br/>401 Not Authorized<br/>406 Invalid Locations<br/>451 Denied<br/>Additional JSON with info on sessions
    Peer->>Meta: If 200, peer sends QUERY request status (request ID, auth code)
    Meta->>Peer: Meta replies with responses:<br/>200 Configured (peer asn, fids, list of sessions)<br/>404 bad request ID<br/>202 waiting<br/>20x waiting for peer to configure<br/>Additional JSON with info on sessions
    loop until both sides report peering complete
        Peer->>Meta: peer sends QUERY request status
        Meta->>Peer: Meta replies with responses:<br/>200 Configured (peer asn, fids, list of sessions)<br/>404 bad request ID<br/>202 waiting<br/>20x waiting for peer to configure<br/>Additional JSON with info on sessions
    end
bgp / autopeer

Add peering API diagram #6
