Add node/protocol developer guide

[zhyatt]

We don't have a central spot for developers looking to help with the node/protocol code itself, it is spread out/buried in a few places:

• https://docs.nano.org/node-implementation/contributing/
• https://docs.nano.org/what-is-nano/living-whitepaper/
• https://docs.nano.org/protocol-design/introduction/
• https://docs.nano.org/node-implementation/introduction/

Consider consolidating the "Node Implementation" and "Protocol Design" top-level nav items into the "Living Whitepaper" section (with collapsed sub sections in left nav) could help consolidate them. Then add "Developer Guide" to top-level with expanded contributing details, etc.

• [ ] #412
• [x] #465
• [ ] #514
• [x] Callout good first issue label in GitHub
• [x] https://github.blog/2019-06-25-assign-issues-to-issue-commenters/

[dsiganos]

If this ticket is primarily due to my feedback, I would like to emphasize that I am not advocating restructuring of existing documentation. I have not read most of the documentation yet and therefore I cannot even begin to consider if it needs restructuring or not.

I am advocating for a "Nano C++ developer quick start guide", which would be supplemental to the existing documentation. It would focus on being a guide with a beginning and an end, rather than being a reference, and it would focus on the typical issues that a typical newbie C++ nano node developer is likely to encounter.

[zhyatt]

If this ticket is primarily due to my feedback

Your feedback echoed some other recent questions so there is a bit of a trend it seems. We want to better support external developers and this seems like an easy change to make to provide better discoverability at least.