Improve Intro to Agile assignment

[adamblanchard]

Based on feedback from the assignment reviews, which found:

1. It unclear what kind of columns are required
2. Not ideal "made up" example projects (e.g. making a cake) which are a little too abstract to apply prioritisation to easily
3. Unclear submission instructions (e.g. just a trello link, or as part of a PR)

The changes I made here include:

1. Linking to the session exercise which has an example board layout that they should use
2. An easier project example to reason about, a personal portfolio, which also helps them think a little more in a technical perspective
3. Clearer submission instructions requiring a PR

The changes can be viewed here https://program.hackyourfuture.dk/~/revisions/PpqYNIm7Z6r95UipRakW/course-content/foundation/intro-to-agile/week-1/assignment