feat: docs, adde particles docs

fix: doc/examples/particles, added web to .gitignore
feat: doc/examples/particles, added more examples,
refactor: Particle does not extend Component
refactor: Particle subclasses in separate folder
refactor: ParticleComponent is now simple container
fix: SingleChildParticle, asserts for child existing
feat: AnimationParticle for Flame Animation
feat: ComponentParticle for Flame Component
feat: SpriteParticle for Flame Sprite

lib/particles/image_particle.dart

@@ -0,0 +1,37 @@
+import 'dart:ui';
+
+import 'package:flutter/foundation.dart';
+
+import '../particle.dart';
+
+/// A [Particle] which renders given [Image] on a [Canvas]
+/// image is centered. If any other behavior is needed, consider
+/// using [ComputedParticle].
+class ImageParticle extends Particle {
+  /// dart.ui [Image] to draw
+  Image image;
+
+  Rect src;
+  Rect dest;
+
+  ImageParticle({
+    @required this.image,
+    Size size,
+    double lifespan,
+    Duration duration,
+  }) : super(lifespan: lifespan, duration: duration) {
+    final srcWidth = image.width.toDouble();
+    final srcHeight = image.height.toDouble();
+    final destWidth = size?.width ?? srcWidth;
+    final destHeight = size?.height ?? srcHeight;
+
+    src = Rect.fromLTWH(0, 0, srcWidth, srcHeight);
+    dest =
+        Rect.fromLTWH(-destWidth / 2, -destHeight / 2, destWidth, destHeight);
+  }
+
+  @override
+  void render(Canvas canvas) {
+    canvas.drawImageRect(image, src, dest, Paint());
+  }
+}