Rework fetch settings, assertion order, doc notes

[atc0005]

Of note:

• move 'Assert source branches are set' check first within 'Assert PR branch is ahead' job as it needs to run and is the cheapest way to fail fast if required values are not set
• simplify code check step where extended options are not needed (see also GH-163 for future work)
• add missing 'Fetch additional references' step for jobs where version calculation is needed (most build & package generation tasks)