Add documentation.

[terabytesoftw]

Add documentation:

• [ ] all widgets.
• [ ] forms.

[samdark]

• [ ] How to use translation.

[vjik]

Need example php code to generate for cases like this:

<input " type="text" name="LocationForm[coords][lng]">

[vjik]

Approximately docs structure:

• [x] The principle of form field configuration:
  • [x] widget config
  • [x] global widget theme config (https://github.com/yiisoft/widget/blob/master/docs/guide/en/themes.md)
  • [x] form field theme
  • [x] passed to widget() config
• [x] Common form fields properties
• [x] Built-in fields
• [x] Input data (principle input data object, built-in PureInputData)
• [x] Form field themes (ThemeContainer configuration, ThemePath usage, built-in themes)
• [ ] Creating own form fields
• [x] PureField helper and PureFieldFactory (don't forget about inheritance)