This PR improves `CONTRIBUTING.md` by replacing some instances of the term `Platform` with `GitHub` for the following reasons:
* This was unnecessary legalese which made the document harder to comprehend.
* Fixes a few ambiguous usages of the term:
* In one usage there is ambiguity if there is a platform called "Platform".
* Ambiguous with other usages of the word when referring to the runtime platform.
* Fixes one incorrect usage of the term, where reference to `"Platform"` was intended to refer to any other platform OTHER THAN `GitHub` or `"the Platform"`.
Typo 1.1
Remove explicit time frame in 6.11.1
change capitalization in 6.11.2 "Primary Developer" is intentionally not defined.
Remove unnecessary "User" 6.11.17
