[docs] Improve documentation and add dartdoc

[aagarwal1012]

• Check/add/update documentation for all public classes and method in the package. Documentation issue address by the Flutter community are mentioned below:

  FadeAnimatedTextKit: fade.dart:5:1
  createState: fade.dart:107:3
  TextLiquidFill: text_liquid_fill.dart:4:1
  createState: text_liquid_fill.dart:71:3
  WavePainter: text_liquid_fill.dart:167:1
  paint: text_liquid_fill.dart:183:3
  shouldRepaint: text_liquid_fill.dart:209:3
  TypewriterAnimatedTextKit: typewriter.dart:6:1
  createState: typewriter.dart:106:3
  ColorizeAnimatedTextKit: colorize.dart:4:1
  createState: colorize.dart:91:3
  RotateAnimatedTextKit: rotate.dart:4:1
  createState: rotate.dart:111:3
  TyperAnimatedTextKit: typer.dart:6:1
  createState: typer.dart:91:3
  WavyAnimatedTextKit: wavy.dart:5:1
  createState: wavy.dart:62:3
  ScaleAnimatedTextKit: scale.dart:5:1
  createState: scale.dart:114:3

• Add dartdoc and host the HTML documentation on github pages.

cc: @awhitford

[aagarwal1012]

@awhitford, hosting dartdoc is just a suggestion for us to make better documentation, it is not necessary for now.

[awhitford]

Generating dartdoc locally is trivial, and you get the API on pub.dev from a publish, so I do not recommend adding the complexity of gh-pages because I expect people will simply go to pub.dev (or rely on their IDE).

[aagarwal1012]

I agree with your argument -- let's close this issue for now.