doc: reorganize the contribution guidelines into a guide
doc: reorganize the contribution guidelines into a guide

Updates #17802

Change-Id: I65ea0f4cde973604c04051e7eb25d12e4facecd3
Reviewed-on: https://go-review.googlesource.com/36626
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Chris Broadfoot <0c2851a4f8ab4aa5606e69c0757771dd13dbe686@golang.org>

polyverse-security/go,derekmarcotte/go,cloudflare/go,ImJasonH/go,golang/go,vsekhar/elastic-go,josharian/go,Cofyc/go,Cofyc/go,cloudflare/go,golang/go,golang/go,vsekhar/elastic-go,CAFxX/go,linux-on-ibm-z/go,cloudflare/go,polyverse-security/go,akutz/go,nawawi/go,ImJasonH/go,ImJasonH/go,Cofyc/go,derekmarcotte/go,codestation/go,linux-on-ibm-z/go,derekmarcotte/go,codestation/go,josharian/go,golang/go,linux-on-ibm-z/go,polyverse-security/go,codestation/go,CAFxX/go,linux-on-ibm-z/go,derekmarcotte/go,codestation/go,CAFxX/go,linux-on-ibm-z/go,linux-on-ibm-z/go,ImJasonH/go,linux-on-ibm-z/go,CAFxX/go,akutz/go,polyverse-security/go,vsekhar/elastic-go,vsekhar/elastic-go,christopher-henderson/Go,codestation/go,josharian/go,golang/go,cloudflare/go,nawawi/go,akutz/go,golang/go,derekmarcotte/go,linux-on-ibm-z/go,josharian/go,CAFxX/go,cloudflare/go,vsekhar/elastic-go,josharian/go,akutz/go,josharian/go,derekmarcotte/go,derekmarcotte/go,CAFxX/go,nawawi/go,CAFxX/go,cloudflare/go,codestation/go,CAFxX/go,golang/go,akutz/go,codestation/go,josharian/go,josharian/go,ImJasonH/go,nawawi/go,Cofyc/go,nawawi/go,cloudflare/go,polyverse-security/go,vsekhar/elastic-go,polyverse-security/go,cloudflare/go,ImJasonH/go,akutz/go,vsekhar/elastic-go,nawawi/go,derekmarcotte/go,Cofyc/go,nawawi/go,golang/go,ImJasonH/go,vsekhar/elastic-go,codestation/go,Cofyc/go,akutz/go,Cofyc/go,Cofyc/go,polyverse-security/go,ImJasonH/go,akutz/go,nawawi/go,polyverse-security/go