chore: Improve instructions for building locally

Based on today's session on setting up Mapeo dev environment, I made the following changes to the Contributing doc:

Clarified the initial steps in the workflow (in developing with Android SDK vs StoryBook), and moved Storybook later down in the doc.
Added links to some of the guides that were shared for setting up dev environment.
Added in additional steps that were not highlighted in the doc, or other insights that could be helpful for developers.

digidem / mapeo-mobile