Split up subjects by learning stage

[vcarl]

Blocked by #245 and #246. Once we have a list of subjects and topics that we feel good about recommending as "what a senior developer needs to know", we should segment it into a series of roadmaps that are small enough and precisely sequenced to be actionable.

Stages:

• Learn to program
• Get a job
• Get a freelance client
• Become a senior engineer