Reorganize contributor guidelines to help new contributors get started

[zenmonkeykstop]

Describe the change

The contributor guidelines at https://developers.securedrop.org/en/latest/contributor_guidelines.html are exhaustive but not ordered to be useful as a guide for new contributors to get started. At the very least, it would be helpful for them to refer folks to the relevant sections in the documentation on setting up environments. Ideally they should present a quickstart guide first, with the information on coding standards and project conventions coming after and presented in a more structured manner,