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/computed_particle.dart

@@ -0,0 +1,32 @@
+import 'dart:ui';
+
+import 'package:flutter/foundation.dart';
+
+import '../particle.dart';
+
+/// A function which should render desired contents
+/// onto a given canvas. External state needed for
+/// rendering should be stored elsewhere, so that this delegate could use it
+typedef ParticleRenderDelegate = void Function(Canvas c, Particle particle);
+
+/// An abstract [Particle] container which delegates renderign outside
+/// Allows to implement very interesting scenarios from scratch.
+class ComputedParticle extends Particle {
+  // A delegate function which will be called
+  // to render particle on each frame
+  ParticleRenderDelegate renderer;
+
+  ComputedParticle({
+    @required this.renderer,
+    double lifespan,
+    Duration duration,
+  }) : super(
+          lifespan: lifespan,
+          duration: duration,
+        );
+
+  @override
+  void render(Canvas canvas) {
+    renderer(canvas, this);
+  }
+}