feat(Button): Deprecate split button dropdowns

This MR removes all mentions and recommended guidelines for using split buttons/dropdowns. See why below.

Why

• Seldom used. See the audit issue, where there is only 1 instance of a split dropdown button.
• Similarity to regular dropdowns, but behave differently.
• Too many patterns for hiding "additional actions". Currently, as you can see in the audit issue, most instances where we hide additional actions, we are putting them in an ellipsis dropdown button (should be added to docs in a follow up).
  • This is the primary reason for deprecating. The inconsistency of how we hide additional actions.
• Ultimately, this is to help simplify our UI and reduce complexity of our UI patterns. This should help reduce cognitive load and help with consistency.

Follow up

• Add the primary method for hiding additional actions in an ellipsis dropdown. Additionally, this should include docs on button groups and when to hide additional actions (This doesn't seem to be documented).

Related:

• #1178.