feat: add library-effects.md intro and summary table (#303)

Author: magnus-madsen

src/library-effects.md

@@ -1,2 +1,12 @@
 # Library Effects
 
+Flix provides several built-in library effects for common I/O operations. These
+effects all have default handlers, so no explicit `runWithIO` is needed in
+`main`.
+
+| Effect                          | Module                    | Description                                                                                      |
+|---------------------------------|---------------------------|--------------------------------------------------------------------------------------------------|
+| [Assert](./assert.md)           | `Assert`                  | Runtime assertions (`assertTrue`, `assertEq`, etc.) with configurable handlers.                  |
+| [Console](./console.md)         | `Sys.Console`             | Terminal I/O: reading input, printing to stdout/stderr, prompts, and menus.                      |
+| [Http / Https](./http-https.md) | `Net.Http`, `Net.Https`   | Sending HTTP requests with a fluent API, middleware (retries, rate limiting, circuit breakers).   |
+| [Process](./process.md)         | `Sys.Process`             | Spawning and managing OS processes.                                                              |