Update Packages development docs

Problem to solve

During the Nuget Retrospective, several helpful sections have been identified and could be added to the packages contribution docs (https://docs.gitlab.com/ee/development/packages.html):

• Add more details on the upload route/endpoint and how it should be investigated to determine the correct encoding.
• Emphasize that any work on workhorse must be started as soon as possible. This is to counter that GitLab Workhorse project has a slower review and release cycle.
• Add a section for the different supported authorization mechanism and their possible customization.
• Emphasize that a feature flag is mandatory.
• Add a section for the Package manager investigation. How to do it, what should it contain.