fix: wait to initialize homepage components with dom dependencies

[radium-v]

Description

This commit resolves a console error on the homepage and addresses DOM ordering and stacking for fixed elements.

Motivation & context

These issues were introduced with #3358 since the DOM is now loaded after the bundle.

Issue type checklist

• Chore: A change that does not impact distributed packages.
• Bug fix: A change that fixes an issue, link to the issue above.
• New feature: A change that adds functionality.

Is this a breaking change?

• This change causes current functionality to break.

Adding or modifying component(s) in @microsoft/fast-components checklist

• I have added a new component
• I have modified an existing component
• I have updated the definition file
• I have updated the configuration file

Process & policy checklist

• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.

[EisenbergEffect]

Interesting. My guess is that the bug was due to the streaming nature of HTML. Does that sound right?

[radium-v]

@EisenbergEffect The <background-design> needs to watch the #hero with an IntersectionObserver - same with the <site-side-navigation> component, which needs to know how many <section>s exist to generate correctly. I'm thinking now that a better solution may be to move these observers outside of the components themselves and into a more general-purpose page script.