Make onboarding of new developers easier

Originally created by @Anonymous on #15864 (Redmine)

We need to define how to do that first. Initial ideas coming from the experience of onboarding new FT members in 2018:

• improve doc for new code contributors (too vague to be actionable)
• ISO build troubleshooting doc: issue → solution/workaround
• clarify expectations e.g. build an ISO to test your proposed changes
• #17781 (closed)
• #17674 (closed)
• #17801 (closed)
• #17802 (closed)
• #16355 (closed)

Team: foundations team; assist: ignifugo

Related issues

• Related to #14516 (closed)
• Related to #7036 (closed)
• Related to #15342 (closed)
• Related to #8477 (closed)
• Related to #16639
• Related to #16355 (closed)
• Related to #16617 (closed)
• Related to #16959 (closed)
• Related to sysadmin#15878 (closed)
• Related to #15349 (closed)