docs: drop redundant Background subheader in Motivation

Motivation content reads directly under the section header.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: scotwells

docs/enhancements/branded-error-pages/README.md

@@ -7,7 +7,6 @@ stage: alpha
 
 - [Summary](#summary)
 - [Motivation](#motivation)
-  - [Background: what a customer sees when something breaks](#background-what-a-customer-sees-when-something-breaks)
   - [Goals](#goals)
   - [Non-Goals](#non-goals)
 - [Proposal](#proposal)
@@ -44,8 +43,6 @@ operator release, while guaranteeing a valid page even if the override is absent
 
 ## Motivation
 
-### Background: what a customer sees when something breaks
-
 Datum's network services route a visitor's request through edge Envoy to a
 customer's backend — either directly, or, for the **Connector**, through an
 outbound tunnel to a private origin. When that path can't be completed, Envoy