Incorporating Amira's GitHub repository

[k8hertweck]

Repo will be available at https://github.com/amira-khan/zipf , which has been added to links.md. I've retained the first attempt at Amira's repo here: https://github.com/amira-khan/zipf_first_draft

Also includes a preamble in solutions.md that addresses #504 . I'll try to make adjustments to individual answers as I proceed.

Remaining questions:

• Do we want this to point to the version 1 tag, or do we want to verbally say that in the narrative? I like the latter, but this means we'll actually need to explain what tags are in the advanced git chapter.
• Mention of this repo as a resource has been added in solutions and the tree appendix. Not sure if we want to include it in Getting Started (the tree appendix is already linked there), or in the Git section.

[k8hertweck]

Opening this for review, though I haven't actually added references to Amira's repo yet...this represents changes I've noticed in the book that seem necessary before starting to track changes. Hoping for review on this while I'm continuing to build the other repo...