Remove nav container from header component example

[aduth]

🛠 Summary of changes

Improves header component markup example to avoid <div class="usa-nav-container">. It also updates the logo link to always point to the root regardless of {{ site.baseurl }} value (previously it would often link to the same page).

Why?

• This markup was originally sourced from identity-site, but the default USWDS component guidance for extended header does not include it
• The usa-nav-container creates additional horizontal padding that causes it to be misaligned with page content that includes navigation

📜 Testing Plan

Easiest to review with whitespace changes hidden, since the changes amount to removing a single wrapper element <div class="usa-nav-container">.

1. npm start
2. Visit http://localhost:4000/banner/
3. Observe reduced horizontal padding on header

👀 Screenshots

Before	After

[aduth]

Visual regression failure is catching the expected diff on the header appearance: