Project setup tutorial

[jcalcaben]

This PR is a:

⬜️ Bug fix 🔳 New topic ⬜️ Content rewrite

Summary

When this Pull Request is merged, it will create content that describes how to set up a PWA theme project using the pwa-buildpack.

It also improves how the side nav looks and functions.

Additional Information

The tutorial should be reviewed in the following order:

• src/pwa-buildpack/project-setup/index.md
• src/pwa-buildpack/project-setup/create-theme-files/index.md
• src/pwa-buildpack/project-setup/install-dependencies/index.md
• src/pwa-buildpack/project-setup/create-configuration-files/index.md
• src/pwa-buildpack/project-setup/create-simple-peregrine-app/index.md

This PR closes GitHub Issue #6

[zetlen]

Hey @jcalcaben, please forgive me if I'm out of touch. Beyond this round of corrections, is there anything I need to to clarify in my requests? Or is this PR out of date? Or would it be best for us to meet personally on this and pair merge it?

[jcalcaben]

@zetlen I've made all the changes you requested, so I just need to resolve the merge conflicts and publish.