Add minimum RSA key size of Code Signing private key to documentation

[PaperMonster]

I found out that AppCenter CLI 2.13.0 and later cannot release updates signed with private key of length lower than 2048 bit. I encountered this problem after upgrading CLI to the latest version with my 512-bit key I created few years ago when 512 was the default value. This was due to a breaking change in jsonwebtoken v9 that rejects RSA keys shorter than 2048 bit.

Even though the default key size is now 2048 when being generated, it might save someone's time in the future to mention the minimum key size in the documentation. (Since the CLI simply returned exit code 3 and debug mode vaguely said "The specified signing key file was not valid", so considerable amount of time spent in investigation)

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Document Details

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• ID: a2c2bd47-97e3-abc8-fbdd-305f0c631827
• Version Independent ID: 318718e7-a38a-7cc5-01a4-7ce0153c4398
• Content: Releasing CodePush updates using the App Center CLI - Visual Studio App Center
• Content Source: docs/distribution/codepush/cli.md
• Service: vs-appcenter
• Sub-service: app-center-distribute
• GitHub Login: @lucen-ms
• Microsoft Alias: lucen