Enabling Markdown and other JSDoc improvements

This change enables GitHub flavored markdown for APIdoc
comments. The code example in map.js shows how to use markdown
for code snippets. doc/index.md is now included again as start
page for the docs. Everything in the doc/tutorials directory
will also be added to the docs as tutorial. As an example, I
moved the ol3.md file with the architecture to the tutorials
directory.

Currently properties and methods annotated with @inheritDoc
or @override won't be documented at all. This is a known issue,
so I added a custom JSDoc plugin with a hack to avoid this.

doc/conf.json

@@ -0,0 +1,26 @@
+{
+    "opts": {
+        "recurse": true,
+        "template": "doc/template",
+        "tutorials": "doc/tutorials"
+    },
+    "tags": {
+        "allowUnknownTags": true
+    },
+    "source": {
+        "includePattern": ".+\\.js(doc)?$",
+        "excludePattern": "(^|\\/|\\\\)_"
+    },
+    "plugins": [ "plugins/markdown", "doc/plugins/inheritdoc" ],
+    "markdown": {
+        "parser": "gfm"
+    },
+    "templates": {
+        "cleverLinks": false,
+        "monospaceLinks": false,
+        "default": {
+            "outputSourceFiles": true
+        }
+    },
+    "jsVersion": 180
+}