Enhance description of working with private packages

[timholy]

• expand on the different roles of different SSH keys
• describe both packages registered with HolyLabRegistry as well as unregistered private packages
• remove last vestiges of Travis (update description of PkgTemplates)
• default to hosting on HolyLab rather than in personal accounts
• fix formatting

[timholy]

There are a lot of tricky whitespace changes, but that makes it harder to focus on the content. It's recommended to review this by clicking on Files changed -> gears icon -> check Hide whitespace -> click Apply and reload

[kdw503]

Much clearer than before.
Thank you.