Refactor and update onboarding for all roles

Changes proposed in this pull request:

This PR improves our onboarding in a variety of ways.

Create a new "general" onboarding template with onboarding items that all team members must perform, and rework the other "role-specific" tickets to remove the common elements.
- This results in a process change: When a new person joins the team, a general onboarding ticket should be created as before, following the same instructions still included at the top of the ticket. However, depending on their role, a second ticket may need to be created. There is a space to paste a link to the first ticket so there is a connection between them.
- h/t @apburnes for the two-issue idea!
Developers now install (nearly) all CLI tools at once using a Brewfile and brew bundle install. See the brew manpage.
Use the collapsable <details> element to hide by default some checklist items that do not apply to all new staff.
Refresh a variety of onboarding resources that had fallen out of date.
Normalize Markdown formatting using Prettier

The "refactored" onboarding does not make public any previously private resources.