Write the Software Packs and NicApp CRD explainer page

[khuyentran1401]

Goal

Explain what a Software Pack is, how it integrates with NKP, and what role the NicApp Custom Resource plays. Conceptual level, not a build guide.

Scope

New explanation page(s) under docs/docs/explanations/ covering:

• What a pack is: a curated collection of open-source tools packaged as ArgoCD applications with a NicApp CRD wrapper.
• Why packs exist: composable alternative to Classic's monolithic bundle; teams install only what they need (Data Science pack, Model Serving pack, etc.).
• The NicApp CRD: declares a pack to the Nebari Operator so it can auto-wire routing (Envoy Gateway), authentication (Keycloak), TLS (cert-manager), and landing-page registration. No manual wiring required.
• Pack lifecycle: install via GitOps/ArgoCD, operator reconciles NicApp resources, pack registers itself with the landing page dynamically.
• Distribution channels: official packs (OpenTeams, in central registry), customer packs (internal), community packs (separate listing, not central registry).
• Relationship to the Nebari Operator: operator watches NicApp resources; without it, packs are just ArgoCD apps.

Out of scope

• Building a pack (pointer lives in catalog sub-issue).
• NicApp CRD field-by-field reference (separate reference sub-issue).
• Per-pack documentation (packs own their own docs).

Acceptance criteria

• Readers can explain what a pack is and why it exists after reading.
• NicApp CRD's role is clear without requiring CRD schema knowledge.
• Distinction between official / customer / community packs is explicit.
• Reader understands that the Nebari Operator is what makes packs "just work".
• Page links out to the real upstream repos so readers can inspect / contribute.

Resources

Most authoritative (code-audited): AGENTS.md in nebari-infrastructure-core (PR #297).

Repos:

• Nebari Operator
• nic CLI
• Software pack template

Example packs:

• LLM serving pack
• Superset pack
• KServe deployment

Design docs in nebari-infrastructure-core (some claims may be aspirational and not yet shipped; cross-check against AGENTS.md):

• Nebari Operator design