rework landing page for the dev guide to flatten logical hierarchy
rework landing page for the dev guide to flatten logical hierarchy

cloudfoundry/docs-dev-guide,krujos/docs-dev-guide,bridgetkromhout/docs-dev-guide,summerzhao/docs-dev-guide,shalako/docs-dev-guide,cwest/docs-dev-guide,logit-io/docs-dev-guide