Update docs for shape optionality and T? nullability Closes #30

[binaek]

Summary

• Aligns documentation with the language change: optionality on the binding (field?), nullability on the type (T?), and removal of !-based shape field forms.
• Refreshes examples across reference pages, TypeScript module samples, and policy-pack structure docs so they no longer teach legacy syntax.

What this PR does

• Rewrites the shapes reference to document the four-way matrix (field: T, field?: T, field: T?, field?: T?), absent vs null behavior, migration from old bang syntax, and an Elvis example with optional fields.
• Adds a nullable types section to types-and-values (string?, containers, shape refs).
• Updates facts and policy reference to describe nullable fact types (fact x: T?, fact x?: T?) instead of “facts are always non-null.”
• Updates the language reference index shape section and fixes getting-started notes on facts.
• Applies the same syntax conventions to fan-out examples (built-ins, rules, TypeScript module pages, etc.).

Changes by area

Core reference

• src/content/docs/reference/shapes.md
• src/content/docs/reference/types-and-values.md
• src/content/docs/reference/facts.md
• src/content/docs/reference/index.md
• src/content/docs/reference/policies.md
• src/content/docs/getting-started/writing-your-first-policy.md

Examples and modules

• Arithmetic, boolean, built-in functions, rules, trinary, using-typescript, structure-of-a-policy-pack.
• typescript_modules/index.md and per-module sentrie/*.md example snippets.

Review notes

• Spot-check that no page still states bare T allows implicit null on shape fields.
• Cross-link shapes ↔ types-and-values ↔ facts for consistent wording on undefined vs null where relevant.

Testing notes

• Documentation-only; preview site build if your pipeline requires it.

Dependency changes

• None.

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