docs: restructure into topical sections and add the four most-asked-for pages (#3044)

docs/handlers/index.md

@@ -0,0 +1,28 @@
+# Inside your handler
+
+A handler's arguments come from the client. Everything *else* it can read, and
+everything it can do while it runs, is here.
+
+What it can read:
+
+* **[The Context](context.md)** is the one extra parameter any handler can
+  ask for: the live request, its headers, its session, and the progress and
+  change-notification verbs.
+* **[Dependencies](dependencies.md)** are parameters the model never sees,
+  filled in by your own functions with `Resolve`.
+* **[Lifespan](lifespan.md)** covers state your server builds once at
+  startup, and how a handler reaches it through the `Context`.
+
+What it can do while it runs:
+
+* Ask the user for more input with **[Elicitation](elicitation.md)**, and
+  **[Multi-round-trip requests](multi-round-trip.md)**, the 2026-07-28
+  pattern that carries it.
+* Report **[Progress](progress.md)** on something slow.
+* Write logs (to standard error, for whoever operates the server) with
+  **[Logging](logging.md)**.
+* Tell subscribed clients that something changed with
+  **[Subscriptions](subscriptions.md)**.
+
+If you haven't registered a handler yet, start with
+**[Tools](../servers/tools.md)**. Every page here assumes you have one.