Changes

Adds explanation of folders in the project structure.
Adds basic architecture diagram
Removes Svelte from Architecture section since we've decided to use vanilla JS as long as we can and leave selecting a JS framework flavour to the developers using Head Start for their project.

Associated issue

Resolves #20

[x] I have performed a self-review of my own code
[x] I have made sure that my PR is easy to review (not too big, includes comments)
~~[ ] I have added/updated tests to prove that my feature works (if not possible please explain why)~~
[x] I have made changes to the README and if the change affects the project setup (npm commands changed, new service added, environmental variable added)
~~[ ] I have added a decision log entry if the change affects the architecture or changes a significant technology~~ (to do in next PR introducing the decision log altogether)
[x] I have notified a reviewer

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