Resource Pages, Home Page, and Header Updates

[lauraherring]

related issue: https://github.com/GSA-TTS/FAC/issues/3369

This PR covers a lot of significant changes to the static site. Most notable are:

• updates to the site navigation
• creating new hub pages for Audit Search resources, Audit Submission resources, and API resources

[James-Paul-Mason]

@jperson1 @lauraherring Here are just a few content revisions/suggestions https://docs.google.com/document/d/1ZJY8vlLwqtPS_LKTbhpW9VVXPJy7WvLak9R5N0AuctI/edit

[jperson1]

@James-Paul-Mason, I've made the changes highlighted in the doc and hit the "resolve" button on the comments.