docs(GlDropdown): Add header to search example

What does this MR do?

In the Gitlab project, we often pair the dropdown search with a header to show what we are searching. This tiny MVC extends the new dropdown docs to have the example match that design.

Does this MR meet the acceptance criteria?

Conformity

• x ] 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.