rich text widget documentation

See merge request !17

# Conflicts:
#   source/import_documentation/Widgets/Reference.rst

attribute enum added

See merge request !20

Remove duplicated line

See merge request !18

Update solid-form doc with min,max attributes

See merge request !16

update: 0.12 beta release

See merge request !15

Add next-mixin in the documentation

See merge request !14

# Conflicts:
#   source/index.rst

update: component doc improvements

See merge request !13