[flow][docs] Add Flow for TypeScript Users comparison page

Summary:
Adds a new prominent top-level page at `website/docs/flow-and-typescript.md` that serves as the canonical long-form comparison between Flow and TypeScript.

Changelog: [internal]

Reviewed By: marcoww6

Differential Revision: D105795128

fbshipit-source-id: cea5eaeda34ff8d582fe396730a3f8be888d1345

Author: gkz

website/docs/faq.md

@@ -281,10 +281,17 @@ const array = ['a', 'b']
 export const genericArray: Array<string> = array.map(a => a)
 ```
 
+## I'm coming from TypeScript. What's different?
+
+Flow shares most of TypeScript's surface syntax — generics, conditional and mapped types, `keyof`, `unknown`, `param is T` type guards, `as const`, JSX — and Flow has deliberately aligned spellings with TypeScript over the last several releases (`unknown` replacing `mixed`, `<T extends Bound>` replacing `<T: Bound>`, `as` replacing `(x: T)` casts, `readonly` / `in` / `out` replacing `+`/`-` variance sigils). The substantive divergences are deliberate Flow choices in favor of stronger static guarantees: object types are exact by default, mutable properties and arrays are invariant, methods are contravariant (not bivariant), `as` casts only widen, and type-guard bodies are validated. React is one area where the two diverge: Flow has first-class `component` / `hook` / `renders` syntax rather than modeling components through function types, `forwardRef`, and framework/library patterns.
+
+See [Flow for TypeScript Users](./flow-and-typescript.md) for the full comparison, organized by what transfers cleanly, what looks the same but means something different, what only Flow has, and what only TypeScript has.
+
 ## See Also {#toc-see-also}
 
 - [Refinements](./lang/refinements.md) — how type narrowing works and when it's invalidated
 - [Type Guards](./types/type-guards.md) — custom refinement functions for cases where `typeof` isn't enough
 - [Variance](./lang/variance.md) — why containers of subtypes aren't always compatible
 - [Generics](./types/generics.md) — understanding generic return types and annotation requirements
 - [Annotation Requirement](./lang/annotation-requirement.md) — when and why Flow requires explicit annotations
+- [Flow for TypeScript Users](./flow-and-typescript.md) — full TypeScript-comparison reference