Added sales and developer onboarding to general onboarding page

To keep onboarding topics centralized, and to make sure that all new team members can be aware of the various topics that they or their colleagues should know about, I added the content of the sales onboarding page to this page by entering the topics into the master checklist. I also copy-pasted the developer onboarding information from its native page to this page (will remove links to other page in later steps of editing).