Update Unboxed readme

[Mikerniker]

Updated the readme to follow the Intro to Solana Mobile

• Fixed broken links
• Updated Headings
• Currently, the broken images and some linked lessons have been linked to the external developer content repo since they don't appear to be within this repo. I wasn't sure whether to add the missing images and lessons here or or link them externally to the other repo (which wouldn't follow the Solana contributing guidelines).

[mikemaccana]

Thank @Mikerniker but the README shouldn't duplicate the lesson. The PR we expect as part of the Superteam Bounty is to update the dependencies for the code.