[Docs Site] various typescript improvements

[Cherry]

• Enables type-checking on a few more files, and improves types in some of them as needed.
• Both @types/hast and mdast-util-mdx-expression are added as dev dependencies, but are actually already present due to transitive requirements, so no real new deps.
• Enables type-checking of the worker, like the functions, in CI

Note: There's a weird type mismatch between the `Fetcher` that `this.env.ASSETS` is and what the `generateRedirectsEvaluator` function expects:

Argument of type '{ fetch(input: RequestInfo<unknown, CfProperties<unknown>>, init?: RequestInit<CfProperties<unknown>> | undefined): Promise<Response>; connect(address: string | SocketAddress, options?: SocketOptions | undefined): Socket; }' is not assignable to parameter of type '{ fetch(input: RequestInfo<unknown, CfProperties<unknown>>, init?: RequestInit<CfProperties<unknown>> | undefined): Promise<Response>; connect(address: string | SocketAddress, options?: SocketOptions | undefined): Socket; queue(queueName: string, messages: ServiceBindingQueueMessage<...>[]): Promise<...>; scheduled(...'.
  Type '{ fetch(input: RequestInfo<unknown, CfProperties<unknown>>, init?: RequestInit<CfProperties<unknown>> | undefined): Promise<Response>; connect(address: string | SocketAddress, options?: SocketOptions | undefined): Socket; }' is missing the following properties from type '{ fetch(input: RequestInfo<unknown, CfProperties<unknown>>, init?: RequestInit<CfProperties<unknown>> | undefined): Promise<Response>; connect(address: string | SocketAddress, options?: SocketOptions | undefined): Socket; queue(queueName: string, messages: ServiceBindingQueueMessage<...>[]): Promise<...>; scheduled(...': queue, scheduled

But some type-checking is better than no type-checking.

cc @KianNH @GregBrimble

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