docs(code): merge coding_standards into contribute/code.rst
docs(code): add docs on testing and cleanup

Simplifies PHP guidelines, mentions complexity
Merge pull request #3 from mrclay/ewinslow-docs-code

docs(code): add docs on testing and cleanup
docs(code): fix whitespace warnings
Merge pull request #6721 from ewinslow/docs-code

docs(code): merge coding_standards into contribute/code.rst