docs: add deployment path selector, first successful outcome steps, resolve auth/sizing contradictions, and clean FAQ

[blue4209211]

Summary of Improvements

This PR addresses critical feedback to streamline first-time user onboarding and eliminate contradictions:

1. 3-Card Deployment Selector on Welcome Page (/docs/)

• Added 3 clear visual onboarding cards:
  • Cloud SaaS (5–10 min)
  • Community Self-Hosted (20–30 min)
  • Enterprise Self-Hosted (30–60+ min)
• Accurately clarified licensing (BSL 1.1 for Server, Apache 2.0 for Agents).

2. Standardized "First Successful Outcome" in All Installation Guides

• Server Installation Guide: Added Step 5 to verify first AI-SRE investigation in NuBi with concrete expected results.
• K8s Agent Installation Guide: Added Step 4 to run live pod restart/OOMKill investigations against cluster telemetry.
• Proxy Agent Quick Start: Added Step 6 with concrete database diagnostics query and expected output.

3. Resolved Contradictions & Sizing Ambiguity

• Authentication: Explicitly clarified that Cloud SaaS is passwordless (SSO / magic links), while Self-Hosted Community initializes with an in-cluster bootstrap admin secret (nudgebee-admin-password).
• Agent & Server Sizing: Standardized resource specifications across FAQ.md and installation guides (Core agent: ~200m CPU / 256MB RAM; Bundled Prometheus stack: ~1–2 CPU / 2–4 GB RAM).
• OpenCost Cleanup: Removed remaining OpenCost references from troubleshooting tables and FAQ.
• FAQ Cleanup: Removed stale date claims (e.g. "December 2025") and updated ticketing and telemetry retention descriptions.
• doc-server README: Updated boilerplate Docusaurus 2 text to Docusaurus 3 NudgeBee Documentation.

Verification

• Tested locally with npm run build (0 errors, 0 broken links).

[gemini-code-assist]

Code Review

This pull request updates the NudgeBee documentation for Docusaurus 3, refines the FAQ section with detailed system requirements, licensing, and architecture details, and introduces a structured grid for deployment paths (Cloud SaaS, Community, and Enterprise). It also adds step-by-step instructions for running initial diagnostic investigations with the NuBi AI assistant across different installation guides. The review feedback highlights that several newly introduced internal links use absolute paths (e.g., /docs/installation/server/), which can break link resolution if the documentation is hosted under a subpath, and recommends replacing them with relative paths.