Update guide with new pub instructions

[jeroen-meijer]

Several things have changed since we last created this guide.

• The authors field is no longer used. Verified publishers are used instead. The authors field can be removed.
• There might be other updates to the transfer workflow that need to be included in the guide.

[nohli]

When trying to change the uploader via (as in the README)

flutter packages pub uploader add community@flutter.zone

this is the output:

Package uploaders are no longer managed from the command line.
Manage uploaders from:

https://pub.dev/packages/android_id/admin
pub finished with exit code 1