Update Outdated Developer Documentation & OpenAPI v1 Specification

[benhalpern]

This PR updates the Forem developer documentation to bring it up to date:

1. Updated OpenAPI Specs: Synchronized api_v1.json with the latest live version from the official Forem repository, upgrading spec paths from 33 to 52 endpoints (adds /api/agent_sessions, /api/billboards, /api/organizations, /api/segments, /api/surveys, and /api/trends).
2. Converted Outdated Domains: Replaced absolute links pointing to docs.forem.com and developers.forem.com with robust relative Markdown links for compile-time validation.
3. Removed Redundant Links: Cleaned up the self-referencing link in docs/frontend/instant-click.md.
4. Modernized Setup References: Updated postgresql.md references to target current Ubuntu LTS versions (22.04 / 24.04) and updated PostgreSQL docs URLs to current.
5. Rewrote CI/CD Documentation: Updated deploying.md to document the current GitHub Actions pipelines (ci.yml and cd.yml), detailing KnapsackPro 15-node parallelization and Heroku CD setups.

Verification:

• Successfully compiled the production build using yarn build.
• Ran linter validation via yarn run api-docs:lint.

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