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/paint_particle.dart

@@ -0,0 +1,43 @@
+import 'dart:ui';
+
+import 'package:flutter/foundation.dart';
+
+import '../components/mixins/single_child_particle.dart';
+import '../particle.dart';
+import 'curved_particle.dart';
+
+/// A particle which renders its child with certain [Paint]
+/// Could be used for applying composite effects.
+/// Be aware that any composite operation is relatively expensive, as involves
+/// copying portions of GPU memory. The less pixels copied - the faster it'll be.
+class PaintParticle extends CurvedParticle with SingleChildParticle {
+  @override
+  Particle child;
+
+  final Paint paint;
+
+  /// Defines Canvas layer bounds
+  /// for applying this particle composite effect.
+  /// Any child content outside this bounds will be clipped.
+  final Rect bounds;
+
+  PaintParticle({
+    @required this.child,
+    @required this.paint,
+
+    // Reasonably large rect for most particles
+    this.bounds = const Rect.fromLTRB(-50, -50, 50, 50),
+    double lifespan,
+    Duration duration,
+  }) : super(
+          lifespan: lifespan,
+          duration: duration,
+        );
+
+  @override
+  void render(Canvas canvas) {
+    canvas.saveLayer(bounds, paint);
+    super.render(canvas);
+    canvas.restore();
+  }
+}