doc(netxlite): revamp the documentation (#523)

Part of https://github.com/ooni/probe-cli/pull/506. In parallel with
tutorials, we also need to make sure we have good documentation.

internal/tutorial/netxlite/README.md

@@ -2,9 +2,34 @@
 
 Netxlite is the underlying networking library we use in OONI. In
 most cases, network experiments do not use netxlite directly, rather
-they use abstractions built on top of netxlite. Though, you need to
-know about netxlite if you need to modify these abstractions.
+they use abstractions built on top of netxlite (e.g., measurex).
+Though, you need to know about netxlite if you need to modify
+these abstractions.
 
 For this reason, this chapter shows the basic netxlite primitives
 that we use when writing higher-level measurement primitives.
 
+We will start from simple primitives and we will combine them
+together to reach to the point where we can perform GET requests
+to websites using already existing TLS or QUIC connections. (The code
+we will end up writing will look like a stripped down version of
+the measurex library, for which there is a separate tutorial.)
+
+Index:
+
+- [chapter01](chapter01) shows how to establish TCP connections;
+
+- [chapter02](chapter02) covers TLS handshakes;
+
+- [chapter03](chapter03) discusses TLS parroting;
+
+- [chapter04](chapter04) shows how to establish QUIC sessions;
+
+- [chapter05](chapter05) is about the "system" DNS resolver;
+
+- [chapter06](chapter06) discusses custom DNS-over-UDP resolvers;
+
+- [chapter07](chapter07) shows how to perform an HTTP GET
+using an already existing TLS connection to a website;
+
+- [chapter08](chapter08) is like chapter07 but for QUIC.