[ffigen] Add more Objective-C documentation

[liamappelbe]

Fixes #1510
Fixes #1589
Fixes #1588

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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[coveralls]

coverage: 89.76%. remained the same
when pulling d03e9d7 on ffigen_docs
into 81ff644 on main.

[goderbauer]

LGTM

[goderbauer]

Two overall comments:

• maybe add a docs/README.md to this package that includes an index of all available docs in the do directory, similar to what Daco did in https://github.com/dart-lang/native/pull/2705/files#diff-a0842ea73f3a116eff7958fbf833dd2280c366c0d8985aa05f8e1403e0f39273
• Can you include a link from the toplevel package README to the doc directory to make these a little easier to find?

[goderbauer]

Consider cross-linking this file with the "Dealing with Objective-C API versioning" file and vice versa. Due to the similarity in the title I can see some people accidentally ending up here even though they are interested in the other one.

[liamappelbe]

Good point. Probably better to just combine them in that case.

[liamappelbe]

Looking at pkgs/ffigen/README.md, the big FAQ section should probably be pulled out into the docs directory. I think it's only in the readme because it predates the docs directory. It makes the top level readme too long, and it's not very discoverable. Some of the longer answers can also be split out into their own doc file.