Forms: Add Docs to Form Section

The form docs are missing examples of input fields.

The following examples need to be added to page: https://design.gitlab.com/components/form

A lot of these will first require creating examples in the GitLab UI repo

• Layout (vertical) form https://design.gitlab.com/components/form#layout

• Layout (horizontal) form https://design.gitlab.com/components/form#layout

• Label (sort by dropdown) https://design.gitlab.com/components/form#labels

• Label (horizontally distributed form) https://design.gitlab.com/components/form#labels

• Input (input field types) https://design.gitlab.com/components/form/#input-fields

• Input (disabled input) https://design.gitlab.com/components/form/#input-fields

• Input (inputs of different widths) https://design.gitlab.com/components/form/#input-fields

• Input (predefined widths from specs + 100% width) https://design.gitlab.com/components/form/#predefined-widths

• Text Area https://design.gitlab.com/components/form/#text-area

• Placeholder https://design.gitlab.com/components/form/#placeholders

• Input validation https://design.gitlab.com/components/form/#validation

• Submit button https://design.gitlab.com/components/form/#submit-form-buttons

• Input with optional text: https://design.gitlab.com/components/form/#required-information

• Help Text https://design.gitlab.com/components/form/#help-text

Additional examples are also needed but not yet built. These may need to be broken out into a separate issue:

• Input field character counter (https://design.gitlab.com/components/form/#preventing-text-overlap). See issue (gitlab-org/gitlab-ui#648 (closed)) for progress
• Text area character counter (https://design.gitlab.com/components/form/#preventing-text-overlap). See issue (gitlab-org/gitlab-ui#648 (closed)) for progress
• Inserting Icons into an input field: https://design.gitlab.com/components/form/#icons