Improve lacking documentation

[NickUfer]

As a developer who wants to create filters using Proxy-WASM it is hard to understand the exact lifecycle of filters. It doesn't seem to be documented anywhere.

It would be great if this project had some page like Pingora's Life of a request which shows using a diagram the call tree of all methods:

 graph TD;
    start("new request")-->early_request_filter;
    early_request_filter-->request_filter;
    request_filter-->upstream_peer;

    upstream_peer-->Connect{{IO: connect to upstream}};

    Connect--connection success-->connected_to_upstream;
    Connect--connection failure-->fail_to_connect;

    connected_to_upstream-->upstream_request_filter;
    upstream_request_filter --> SendReq{{IO: send request to upstream}};
    SendReq-->RecvResp{{IO: read response from upstream}};
    RecvResp-->upstream_response_filter-->response_filter-->upstream_response_body_filter-->response_body_filter-->logging-->endreq("request done");

    fail_to_connect --can retry-->upstream_peer;
    fail_to_connect --can't retry-->fail_to_proxy--send error response-->logging;

    RecvResp--failure-->IOFailure;
    SendReq--failure-->IOFailure;
    error_while_proxy--can retry-->upstream_peer;
    error_while_proxy--can't retry-->fail_to_proxy;

    request_filter --send response-->logging


    Error>any response filter error]-->error_while_proxy
    IOFailure>IO error]-->error_while_proxy