docs: edit installation instructions to match new example structure

[cadenmyers13]

Problem

We are moving all example scripts under a directory corresponding to its pack (ie examples/pdf/ch03NiModelling instead of examples/ch03NiModelling). This means we need to update our pack installation instructions.

Proposed solution

Depending on what we decide in this PR #49, edit the docs accordingly

[cadenmyers13]

@sbillinge Fyi i can't add issues to milestones, can you add this to this upcoming release?