[clang-tools-extra] [clang-doc] refactor JSONGenerator array usage (PR #145595)

[Petr Hosek via cfe-commits]

https://github.com/petrhosek approved this pull request.

Looks great! One idea for future improvement would be to wrap `std::optional<StringRef> RepositoryUrl` in a object (e.g. `SerializationContext`) so in the future if we need to pass around more information beside the URL, we don't need to modify every one of these functions and lambdas.

https://github.com/llvm/llvm-project/pull/145595