General topic for all form components and general form validation

[kdinev]

Create an overarching topic for all "Form components", which should include guidance and examples of how to build template and reactive forms with igniteui-angular form components. The topic should also include validation examples, again for both template and reactive forms.

Example from angular.io forms

Our form components are also not categorized well. We should consolidate them under the same category, which should be called Form Components.

Currently some of the form components live under the "Data Entry & Display" category:

• autocomplete
• button
• checkbox
• input group
• label and input
• mask
• radio
• select
• switch

Others live under the "Scheduling" category:

• date picker
• date-range picker
• date-time editor
• time-picker

Others living in wild places:

• slider - "Interactions"
• combo - "Grids & lists"
• single-select combo - "Combo"

All of the components above are Form Components and should be indicated as such.