Update for signac 2.0 and general cleanup

[vyasr]

Description

This PR updates the documentation for changes in signac 2.0. In the process, I also cleaned up a number of documents to tighten up the writing.

There are a few notes/requests for reviewers:

• All of the example outputs in the tutorial on Stampede2 need to be refreshed since our output format has changed substantially over time. I no longer have access to any actual supercomputer and would appreciate if someone else could run those tutorial scripts real quick and post the updated output.
• There is a lot of duplication between the project and job pages. I think that the two concepts are too closely intertwined to have two separate pages. We should consider combining the pages to reduce duplication. That doesn't have to be part of this PR, I'll just open an issue if others agree.
• The primary branch on this repo is still master. We should rename to main once this PR is merged.

Motivation and Context

The signac 2.0 release is imminent and we'd like the documentation to be ready to go live when the release happens.

Checklist:

• [x] I am familiar with the Contributing Guidelines.
• [x] I agree with the terms of the Contributor Agreement.
• [x] My code follows the code style guideline of this project.

[vyasr]

Something must have been wrong with my clone of this repo... I checked for the presence of a hooks file and couldn't find one at all. The current file is one that I added from scratch. Not sure what happened. I'll pull the one from the master branch and combine the two.