fix(dropdown button): Differentiate between dropdown panel and button

What does this MR do?

• Includes dropdown button examples in Buttons section so that all variants and configuration can be viewed and tested.
• Removes options from Dropdown sections that only focused on button variants so that all examples focus on changes to the dropdown panel itself.

A key result of this MR is the ability to complete an audit (gitlab-org/gitlab-services/design.gitlab.com#651 (closed)) ensuring that the component library and GitLab UI are in sync as the migration happens.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.