docs: fix formatting in BGP control plane installation docs
docs: fix formatting in BGP control plane installation docs

Add missing title underline and format paragraphs/lists correctly.

Suggested-by: Quentin Monnet <d6b8e48afb2534b213e391cab43016505747a234@isovalent.com>
Signed-off-by: Tobias Klauser <059b8b880f8441509ec8a65b50b4c6ae74ebea76@cilium.io>

cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium,cilium/cilium