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/component_particle.dart

@@ -0,0 +1,35 @@
+import 'dart:ui';
+
+import 'package:flutter/foundation.dart';
+
+import '../particle.dart';
+import '../position.dart';
+import '../components/component.dart';
+
+class ComponentParticle extends Particle {
+  final Component component;
+  final Position size;
+  final Paint overridePaint;
+
+  ComponentParticle({
+    @required this.component,
+    this.size,
+    this.overridePaint,
+    double lifespan,
+    Duration duration,
+  }) : super(
+          lifespan: lifespan,
+          duration: duration,
+        );
+
+  @override
+  void render(Canvas canvas) {
+    component.render(canvas);
+  }
+
+  @override
+  void update(double dt) {
+    super.update(dt);
+    component.update(dt);
+  }
+}