Simplify doc generation infra

PUBLISHED_FROM=80e77fda1c358294f251ca77d3dd64e8ca9160e5

docs/build-options/intro.md

 ---
 title: Build Options
+items:
+  - { type: file, name: enabling-flags.md }
+  - { type: file, name: disabling-flags.md }
+  - { type: file, name: platform-spec.md }
+  - { type: file, name: tunables.md }
 ---
 
 Mongoose source code ships in a single .c file that contains functionality

docs/build-options/items.json

-{
-  "items": [
-    { "type": "markdown", "name": "intro.md" },
-    { "type": "markdown", "name": "enabling-flags.md" },
-    { "type": "markdown", "name": "disabling-flags.md" },
-    { "type": "markdown", "name": "platform-spec.md" },
-    { "type": "markdown", "name": "tunables.md" }
-  ]
-}

docs/design-concept/intro.md

 ---
 title: Design Concept
+items:
+  - { type: file, name: memory-buffers.md }
+  - { type: file, name: event-handler.md }
+  - { type: file, name: events.md }
+  - { type: file, name: conn-flags.md }
 ---
 
 Mongoose is a multi-protocol networking library that implements non-blocking,

docs/design-concept/items.json

-{
-  "items": [
-    { "type": "markdown", "name": "intro.md" },
-    { "type": "markdown", "name": "memory-buffers.md" },
-    { "type": "markdown", "name": "event-handler.md" },
-    { "type": "markdown", "name": "events.md" },
-    { "type": "markdown", "name": "conn-flags.md" }
-  ]
-}

docs/intro.md

+---
+title: Mongoose - a networking library
+color: '#1D6482'
+repo: https://github.com/cesanta/mongoose
+items:
+  - { type: dir, name: usage-example }
+  - { type: dir, name: design-concept }
+  - { type: dir, name: build-options }
+  - { type: flat, name: c-api }
+---
+
+Mongoose is a multi-protocol embedded networking library with
+that implements TCP, UDP, HTTP, WebSocket, CoAP, MQTT, JSON-RPC,
+both client and server mode.

docs/items.json

-{
-  "items": [
-    { "type": "section", "name": "usage-example" },
-    { "type": "section", "name": "design-concept" },
-    { "type": "section", "name": "build-options" },
-    { "type": "flat_sections_set", "name": "c-api" }
-  ]
-}

docs/usage-example/items.json

-{
-  "items": [
-    { "type": "markdown", "name": "intro.md" }
-  ]
-}