Clean up docs/examples - one directory per tutorial, more templating. (#4965)

* Clean up docs/examples - one directory per tutorial, more templating.

* Fix some URLs

* tutorials: update rendered CDN links to 1.0.0 and L.circle signature (#4975)

* Update shown CDN links to 1.0.0

As initially done in PR #4967, but just for the rendered links (what the users sees), as actual link / script tags are already updated.

* DOCS update L.circle to new syntax

in quick-start tutorial, following PR #4974.

* DOCS update L.circle to new syntak (example src)

updated L.circle to new signature (radius as an option instead of 2nd parameter).

* tutorials: use L.geoJSON new syntax (#4983)

* tutorials: use new syntax L.geoJSON

for consistency with recommended factory new syntax (`L.geoJSON` instead of legacy `L.geoJson`).
As done in PR #4933 for docstrings.

* tutorials: L.geoJSON new syntax in script

* Renamed several files to directory/index.md

docs/examples/quick-start/example-overlays.md

@@ -0,0 +1,35 @@
+---
+layout: tutorial_frame
+title: Quick Start
+customMapContainer: "true"
+---
+<div id='mapid' style='width: 600px; height: 400px;'></div>
+<script>
+
+	var mymap = L.map('mapid').setView([51.505, -0.09], 13);
+
+	L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token=pk.eyJ1IjoibWFwYm94IiwiYSI6ImNpandmbXliNDBjZWd2M2x6bDk3c2ZtOTkifQ._QA7i5Mpkd_m30IGElHziw', {
+		maxZoom: 18,
+		attribution: 'Map data &copy; <a href="http://openstreetmap.org">OpenStreetMap</a> contributors, ' +
+			'<a href="http://creativecommons.org/licenses/by-sa/2.0/">CC-BY-SA</a>, ' +
+			'Imagery © <a href="http://mapbox.com">Mapbox</a>',
+		id: 'mapbox.streets'
+	}).addTo(mymap);
+
+	L.marker([51.5, -0.09]).addTo(mymap);
+
+	L.circle([51.508, -0.11], {
+		color: 'red',
+		fillColor: '#f03',
+		fillOpacity: 0.5,
+		radius: 500
+	}).addTo(mymap);
+
+	L.polygon([
+		[51.509, -0.08],
+		[51.503, -0.06],
+		[51.51, -0.047]
+	]).addTo(mymap);
+
+
+</script>