doc: Update macOS-development.md for free Apple Developer accounts (Personal Team)

[JanSndr]

I followed the documentation on how to set up the Nextcloud desktop client for development under macOS (macOS-development.md) and ran into an issue regarding the Apple Developer Team ID.
Under the "Set Your Apple Development Team" section, the guide states:
"You can find your team ID on the Apple Developer Account page under 'Membership details'."

However, users with a free Apple Developer account (Personal Team) do not have the "Membership details" section on the website, making it impossible to find the Team ID there.

Here is the workaround to find the Team ID using Xcode and Keychain Access:

1. Open Xcode > Settings > Accounts.
2. Select your Apple account (or click + to add it).
3. Select your Personal Team and click Manage Certificates....
4. In the bottom-left corner, click the + button and select Apple Development to generate a local development certificate.
5. Open the Keychain Access app on macOS (not the Passwords app).
6. Search for Apple Development and double-click the certificate (not the private key).

7. Look for the Organizational Unit field — this 10-character code is your Apple Developer Team ID.
8. Use this ID for NEXTCLOUD.cmake and Build.xcconfig as described in the guide.

Would it make sense to update macOS-development.md to include this note for contributors using a free Personal Team account? I'd be happy to open a quick PR if desired.

[i2h3]

I have my personal account registered since 16 years and corporate accounts were always through invitations into an existing and paid team membership, so I cannot recall how this used to work when freshly setting up without a developer program membership. This appears to be the safest way to figure out which identifiers to use.

By the way: the team identifiers are public information and not a secret. They also cannot be stolen.