Adds new Zero IA

[ZPain8464]

This PR restructures the docs IA for Zero. It:

• Moves IdP guides, Integrations guides to the Guides sidebar
• Splits up Guides into the following subsections:
  • Upstream Services
  • External Data Sources
  • Courses
  • Identity Providers
  • Configure Pomerium
• Removes Securing TCP-based Services, and adds an additional Postgres example to capabilities/tcp/examples/postgres
• Removes the JS SDK guide
• Removes Local OIDC guide
• Splits up concepts/certificates into an mTLS guide under guides/certificates; the body content from the original page will be repurposed into a resources piece on the Marketing site
• Moves "Core" courses fundamentals to the Guides subsection (will replace with Zero courses in another PR)
• Moves the External Data Sources page to Capabilities
• Adds a Zero Install page with the onboarding instructions for each deployment environment
• Moves nested product directories to the top, including:
  • Core
  • Enterprise
  • Clients
  • Kubernetes
  • Zero
• Moves Cluster Status to the Troubleshooting directory
• Removes "Deploy"
• Fixes broken links and adds redirects

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