Add an Edit button to the Documentation subsite

[jiribenes]

Each page of the Documentation subsite could have an edit field in its header

---
layout: docs
title:  "Examples"
edit: https://github.com/effekt-lang/effekt-website/edit/master/examples.md
---

which renders as some [✏️] button somewhere in order to quickly edit the correct file in the correct repository.

If I recall correctly, GitHub should offer you to create a fork with your changes followed by a PR.
This way, it's easy enough to fix typos/problems on all documentation pages: in case studies, in the upcoming Tour de Effekt, etc.

Scala's documentation has a very similar button:

[jiribenes]

Related to #80: we could not only have an "Edit" button, but also a "Link / Go to Source" button -- that would already help (and it's mostly the same code as this issue!)