Fluid inputs: TextInput docs

[aagonzales]

Fluid inputs: Add TextInput website docs

Prerequisite

• [x] Update text input to use new content template before integrating fluid inputs.
  • https://github.com/carbon-design-system/carbon-website/pull/3115

Tasks

• [x] Create website branch to house all fluid input updates (do not merge)
• [x] Update "style" tab to include Fluid variant
• [x] Update "usage" tab to include Fluid variant
• ~Create fluid input pattern with initial Fluid TextInput~ Fluid input form pattern shall be addressed in a separate issue.