probe-cli: improve release process documentation

[bassosimone]

We want to:

• divide the release check-list in homogeneous blocks: d9a730d 635ed0f 6e4d789 ce0f928 6c254ad
• explain the branching and releasing model: ooni/probe-cli#1533
• document what we should do for each homogeneous block ooni/probe-cli#1534