doc: Replace coding.txt with Bitcoin developer-notes.md

[jamescowens]

This PR is a tracking/discussion PR that starts with replacing our severely outdated coding.txt style guide with Bitcoin's developer notes markdown document. The Bitcoin style is what the core devs have been following in large part with the new code for Gridcoin.

I am marking this as draft, and welcome discussion on this to fine tune it for our blockchain with additional change commits prior to merging.

[jamescowens]

In particular I will start out with referencing a discussion the core devs were having in #2177. @cyrossignol made some good comments which I will repeat below:

As an aside, these are the rough guidelines for managing legacy code style that seem standard in most teams that I've worked on:

• Code in new modules should always follow the style guide
• When modifying legacy code, be consistent with the style of the surrounding scope
  • Typically, a function boundary, but it could be a small file/class or a large nested block
  • Prefer to avoid mixing cosmetic changes with domain changes (batch-commit formatting as a whole separately)
  • New scopes can follow the style guide to ease expected transitions later
• Do not modify the style of upstream code
• Rely on tools for style enforcement rather than manual review
  • Use linters to flag style violations
  • Use formatters to apply style before submission
  • "Policy, not personal"

Of course, we need an up-to-date style guide for any of this to really matter. I think we do a decent job following these points so far (I surely haven't done so absolutely myself). I mention these because formalizing our position should save us time—I'd rather discuss and review style changes as little as possible. No convention or tool set is perfect, though. The goal is to minimize the distraction of code style from the review process—both for new reviews (PRs) and historical (git log/blame, change tracking, etc.)—so I propose these guidelines as a starting point.

Note that bullet number 2 above is different than the stance taken in Bitcoin developer-notes.md in the Coding Style (General) section.

[barton2526]

not applicable?

[jamescowens]

No, I think it is. We should not be using valid Gridcoin address in RPC examples.