Information architecture update

[ethanpalm]

This PR updates the labels and grouping of content in docs.json.

Goal is to group content with meaningful labels and reduce the layers of nesting (for example, integrations go from navigating three levels deep to two levels).

Summary of changes:

• "Core Concepts" -> "Core Configuration": make it more task-based. People need to configure these aspects of their docs, not just learn concepts
• Add more pages to "Guides": content that are discrete tasks people may or may not need to do. Compared to core configurations which we'd expect everyone to do no matter their circumstances
• Make "Integrations", "Authentication and Personalization", and "Version Control and CI/CD" their own groups
• Add icons for pages and groups that didn't have them

I'd originally changed group titles to sentence case, but since this should be site-wide I'm saving it for a future PR. That way it can all be done at once and we don't have a mix of some sentence case and some title case.

[handotdev]

Makes sense to me!