Document crossplane project simulate and crossplane resource simulate #1162

What's Missing?

The CLI is gaining an alpha crossplane project simulate (crossplane/cli#436, for crossplane/cli#367), alongside crossplane resource simulate (crossplane/cli#429, merged). Neither is documented yet. project simulate previews how a project's changed Compositions and embedded functions affect the composite resources on a live cluster. Each composed managed resource can also be planned by its provider's diff server, so cloud-side changes and drift show up.

The docs should cover:

• What it's for: previewing a change in CI or locally before applying it, and how it relates to crossplane diff (crossplane-contrib/crossplane-diff), whose engine it uses.
• Input modes: -f/--input takes a project file (the default), Composition files, or XR/claim files, and the mode follows from what's passed. One path per -f; - reads stdin.
• Provider plans: on by default, --skip-plan to opt out; --providers, --image-configs and --session-id. Kinds no diff server serves keep the cluster diff, and approximate means the server couldn't diff.
• Output formats: -o plan (the default, terraform-style), -o diff, and -o json|yaml with the plan.Result schema: before/after, field changes and their path grammar, composites, errors, the summary.
• Exit codes for CI gating: 0, 1, 2, 3, 80, 130, and their precedence.
• Scoping and pinned composites: --name, -n, --include-pinned, and how XRs on a Manual update policy are reported.
• Timeouts: --timeout as a per-XR budget, and what happens when one XR fails.
• Permissions: the cluster access a CI identity needs; it works with get-only access to Functions, Compositions, XRDs and EnvironmentConfigs.

The CLI surface is alpha and still settling, so I'll open the docs PR after crossplane/cli#436 passes review.