docs: declare table fields with @Field and @Relation decorators in examples

[MrSociety404]

Summary

The table-declaring examples in the docs predate the @Field/@Relation decorators and still teach the legacy style (bare declare fields). Since properties without @Field are omitted from the TableDefinition.fields map handed to the adapter, the examples produced tables with no field definitions.

• @Field(type) added on every column in every example (docs/2.table-definitions.md, docs/3.table-modifiers.md).
• @Relation({ to: () => User }) added on the foreign-key fields (UserActivity.userId, Post.authorId), consistent with the file's own Relation section.
• Intro prose updated: it taught "declare fields with TypeScript's declare keyword" with no mention of @Field.

Only the sections dedicated to @Field and @Relation themselves were already up to date; everything else is aligned on the same conventions (@Index → @Field → @Relation, @Field(\"any\") for object payloads, @Field(\"date\") for dates).

Greptile Summary

This PR updates the table documentation to show the current decorator-based table style.

• Added @Field decorators to table fields in the examples.
• Added @Relation decorators to foreign-key examples.
• Updated example imports and intro text to match the decorator usage.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.

Important Files Changed

Filename	Overview
docs/2.table-definitions.md	Updates table-definition examples with @Field, relation metadata, and the imports needed by the shown snippets.
docs/3.table-modifiers.md	Updates modifier examples so decorated fields are also declared with @Field.

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[MrSociety404]

@greptile review

[MrSociety404]

@greptile review