docs(firestore): Add docs on Firestore x SPM integration

[ncooke3]

The Firestore bits of the Package.swift are justifiably complicated. I vibe coded these docs to explain the current state.

cc: @rafikhan

#no-changelog

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[ncooke3]

/gemini review These docs are meant to educate repo maintainers on the organization of the Firestore-related targets in the Package.swift

[gemini-code-assist]

Code Review

This pull request adds excellent documentation explaining the complex Swift Package Manager target hierarchy for Firestore. The document is well-structured and clearly explains the different build options (binary, source, and CI-local) and the testing setup. I have a couple of minor suggestions to improve consistency and clarity in the dependency diagrams and target descriptions. Overall, this is a great addition that will be very helpful for developers.

[gemini-code-assist]

This description makes a forward reference to the "source-based build" section, which appears later in the document. It would be clearer to provide a self-contained description to improve readability.

Suggested change

	* **`FirebaseFirestoreTarget`**: A wrapper target, same as in the source-based
	build.
	* **`FirebaseFirestoreTarget`**: The wrapper target that provides a stable entry point for clients.

[gemini-code-assist]

The dependency hierarchies for binary and source builds are slightly inconsistent. The source-based build hierarchy includes BoringSSL, which is a transitive dependency of gRPC-cpp, while the binary-based build hierarchy appears to only list direct dependencies. To make the diagrams consistent and easier to understand, it's better to only show direct dependencies in both. This change removes the transitive BoringSSL dependency from this diagram and updates the tree structure accordingly.

Suggested change

	├── gRPC-cpp (source) (from https://github.com/grpc/grpc-ios.git)
	└── BoringSSL (source) (from https://github.com/firebase/boringSSL-SwiftPM.git)
	└── gRPC-cpp (source) (from https://github.com/grpc/grpc-ios.git)

[andrewheard]

Should we document how to set FIREBASE_SOURCE_FIRESTORE since, correct me if I'm wrong, this can only be set before launching Xcode. I haven't tried setting FIREBASE_SOURCE_FIRESTORE in the Environment Variables for the scheme and then running "Reset Package Caches" but I'm fairly sure that wouldn't work.

[ncooke3]

That's a great suggestion! Will do.

[daymxn]

Left some nits, but otherwise lgtm.

Is this something we'd wanna link to from other docs? Like Firestore/README.md?

[daymxn]

nit: Why did it capitalize every word in each header?

Suggested change

	## Main Product
	## Main product

[daymxn]

nit: The repetition of "Swift Package Manager" throughout the docs is a bit verbose.

Suggested change

	The main entry point for integrating Firestore via Swift Package Manager is the
	The main entry point for integrating Firestore via SPM is the

Changing to a comment instead of a +2, so you can enable auto merge and have @cherylEnkidu approve it instead