webtransport: add a page about WebTransport sessions (#33)

content/docs/webtransport/server.md

@@ -18,7 +18,7 @@ The `webtransport.Server` wraps an `http3.Server`. For more details on how to se
 
 Assume a server is running on `example.com`. This code starts an HTTP/3 server on (UDP) port 443. The server can handle regular HTTP/3 requests to `https://example.com`.
 
-To accept the Extended CONNECT request, the application needs to define an HTTP handler. In this example, we want to accept WebTransport sessions at `https://example.com/webtransport`. It is possible to reject an upgrade request by sending a non-2xx status code. Inside the handler, calling `webtransport.Server.Upgrade` accepts the WebTransport session, and it returns a `webtransport.Session`.
+To accept the Extended CONNECT request, the application needs to define an HTTP handler. In this example, we want to accept WebTransport sessions at `https://example.com/webtransport`. It is possible to reject an upgrade request by sending a non-2xx status code. Inside the handler, calling `webtransport.Server.Upgrade` accepts the [WebTransport session]({{< relref "session.md" >}}), and it returns a `webtransport.Session`.
 
 ```go
 s := webtransport.Server{

content/docs/webtransport/session.md

@@ -0,0 +1,38 @@
+---
+title: WebTransport Session
+toc: true
+weight: 3
+---
+
+A WebTransport Session functions similarly to a [QUIC Connection]({{< relref "../quic/connection.md" >}}), enabling the opening and accepting of streams, as well as the sending and receiving of datagrams. 
+
+The API of `webtransport.Session` is _almost_ identical to that of `quic.Connection`, with a few minor differences: For example, QUIC allows streams to be reset using a 62-bit error code, whereas WebTransport limits the error code range to 32 bits.
+
+## Closing a WebTransport Session
+
+The WebTransport session can be closed by calling the `CloseWithError` method:
+```go
+sess.CloseWithError(1234, "please stop talking to me 🤐")
+```
+
+Similar to closing a `quic.Connection`, this action causes all calls to `AcceptStream` and `OpenStream`, as well as stream `Read` and `Write` calls, to return immediately.
+
+{{< callout type="warning" >}}
+  `CloseWithError` only closes the WebTransport session, but not the underlying QUIC connection.
+{{< /callout >}}
+
+On the receiver side, this error will be surfaced as a `webtransport.SessionError`:
+```go
+var sessErr *webtransport.SessionError
+if errors.As(err, &sessErr) {
+  errorCode := sessErr.ErrorCode
+  errorMessage := sessErr.Message
+}
+```
+
+Additionally, the underlying QUIC connection might close for various reasons, potentially triggering any of the errors detailed in the [error assertion section]({{< relref "../quic/connection.md#error-assertion" >}}).
+
+
+## 📝 Future Work
+
+* WebTransport Datagrams: [#8](https://github.com/quic-go/webtransport-go/issues/8)