Document the Crossplane project file (and the new pre-built function and per-language schema options)

[negz]

What's Missing?

The Crossplane CLI's project workflow (crossplane project init/build/run) is configured by a project file — a Project resource whose spec (ProjectSpec) controls the repository, dependencies, paths, target architectures, embedded functions, and schema generation. Today this file has no prose documentation. The only place any of it surfaces is the auto-generated CLI command reference, which documents command flags but not the project file's schema. The packaging docs (Configurations) only cover the older crossplane xpkg build + crossplane.yaml flow, not the newer Projects flow.

crossplane/cli#24 just merged and adds two new optional ProjectSpec fields that need documenting:

1. Pre-built function runtimes (spec.functions) — fixes crossplane/cli#21. An explicit list of functions to build, which disables the default per-subdirectory auto-discovery. Each entry uses a source discriminator (Directory or Tarball). Tarball sources skip language detection and load a pre-built single-platform OCI image tarball per target architecture (<pathPrefix>-<arch>.tar/.tar.gz), so projects can build functions with their own toolchain (make, Nix, Bazel, ko, docker save, etc.).

spec:
  architectures: [amd64, arm64]
  functions:
    - source: Directory
      directory:
        name: function-a
    - source: Tarball
      tarball:
        name: function-b
        pathPrefix: build/function-b

2. Per-language schema generation (spec.schemas.languages) — fixes crossplane/cli#29. Restricts schema generation to a subset of the supported languages (Go, JSON, KCL, Python) instead of generating all four. Applies to both the project's own XRDs and its dependencies, and flows through project build/run and dependency update-cache/clean-cache.

spec:
  schemas:
    languages: [python]

Ideally these are documented as part of a broader project-file reference page rather than in isolation, since the project file itself is currently undocumented.

Note the CLI docs are mid-restructure (#1104 moves them into their own versioned tree and gives them a separate sidebar/version menu), so placement should account for that.