[TF-M] Minor updates to the tf.org project maintenance

Hello TF-A and TF-M communities,

I've done 2 minor updates to the generic tf.org project maintenance process, which TF-A and TF-M both use. Please see the pull request here:

https://github.com/TrustedFirmware/tf_docs/pull/1

Please provide your feedback on the pull request, if any.

Happy end-of-year celebrations to all!

Best regards, Sandrine