docs: create explanation docs — execution model, security, workspaces, resolution

[vdemeester]

Context

Only 2 explanation docs exist (both about deprecated PipelineResources). No user-facing "why" documentation exists. Users can learn how to use Tekton but not why it works the way it does. See Diátaxis audit for full analysis.

Part of #9740.

Explanations to create

File	Answers	Linked from
explanation/execution-model.md	Why Tasks = Pods, step ordering via entrypoint, reconciliation loop, what happens when a TaskRun is created	All Task/TaskRun how-tos + tutorials
explanation/when-to-use-what.md	Task vs Pipeline vs StepAction — decision guide with tradeoffs	tutorials/first-task, tutorials/first-pipeline, how-to/create-stepactions
explanation/workspaces-and-data.md	Why workspaces exist, PVC lifecycle, emptyDir vs PVC vs CSI tradeoffs	All workspace how-tos + tutorials/using-workspaces
explanation/resolution-model.md	Why remote resolution, how resolvers work, trust model	Resolver how-tos + how-to/run-a-task (remote Tasks section)
explanation/security-model.md	ServiceAccounts, RBAC, credential scoping, hermetic builds, trusted resources	Auth how-tos + how-to/run-with-restricted-pod-security

Linking pattern

Explanations are standalone — reachable from any doc type but not part of a linear sequence:

Each explanation includes:
  📘 "Learn by doing" → link back to relevant tutorial
  🔧 "Apply this" → links to relevant how-tos
  📖 "Look it up" → link to reference

Content sources

Some content can be drawn from existing developer docs:

• developers/controller-logic.md → execution-model
• developers/taskruns.md → execution-model (entrypoint rewriting)
• developers/affinity-assistant.md → workspaces-and-data
• developers/results-lifecycle.md → execution-model

The rest needs to be written fresh, aimed at users (not contributors).

Acceptance criteria

• Each explanation answers "why" and "how does this work" — not "how to do X"
• Written for users, not contributors
• Links to tutorials, how-tos, and reference for practical application
• Reviewed for technical accuracy by maintainers