Contribution guide consolidation

[kattni]

• Creates customisable versions of code and docs contribution guides
  • Requires changes to config.yml to use in a target repo.
• Adds a customisable feature request explanation guide
• Updates docs-tools testbed to include the guides.
• Cleans up code and docs guides.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[kattni]

I'm not adding TODO to the spelling word list, so the builds will fail until you complete the TODOs in the feature request guide.

[freakboy3742]

I've just pushed updates from a review of the new content; this is looking really good.

For the benefit of other observers - Kattni and I have spoken in person about some next steps, including some additional re-organization of this content prior to integrating into Toga, Rubicon and Briefcase.