SE-384 Add details and clarify roles for newcomers and mentors

This PR updates the roles and expectations for mentors and newcomers, and provides details on the onboarding and evaluation process.

What's been done here

• Adds a new page, Onboarding & Evaluation Process, which is linked from the main menu. This page provides an overview of the newcomer's trial period broken into weeks with the evaluation steps and some progress expectations at each stage.

  Evaluation criteria are also specified, moved from Roles#Mentor and clarified.

  The onboarding process overlays the sprint planning process, and so may or may not align with sprint weeks.

• Adds more information about creating Newcomer-friendly tasks, including a minimum amount of information that must be provided on such a task. Newcomers are encouraged to enforce this minimum information set too.

• Adds more detail and specific expectations to the Newcomer and Mentor roles.

• Updates the expectations related to newcomer-friendly tasks for core team members, sprint prioritization, and epic owners.

What hasn't been done here

(but may be done as part of a future task)

• Should we standardize the metrics we use to assess candidates? We haven't really decided how far down the "standardization" route we want to go with our evaluations. But where specific requirements exist (like minimum spillover), I felt it worth mentioning clearly to support transparency and fairness for our candidates.
• How to ensure candidates have enough variety of tasks to allow us to effectively assess their abilities? Some improvements have been made here with the changes to role expectations, but we'll need to continuously improve on this.

JIRA tickets: SE-384

Discussions: See SE-384 for relevant forum and task links.

Testing instructions:

1. Read through the markdown changes and ensuring the text is accurate and renders correctly.
2. Build the docs to ensure that the addition to the table of contents works correctly. See README for instructions -- you'll need pipenv and mkdocs installed.

Reviewers:

• @symbolist

And anyone else who would like to comment is welcome! I'm unsure how many people need to approve before this can be merged.