docs(sessions): 📝 add summary section (#72)

## Description

This PR adds a summary section to the “Working with files” session.

Closes #60 

<!-- Please delete as appropriate: -->
This PR needs an in-depth review.

## Checklist

- [X] Formatted Markdown
- [X] Ran `just run-all`

sessions/working-with-files.qmd

@@ -73,7 +73,8 @@ Here are some tips for good naming practices:
 
 `.md` stands for
 [Markdown](https://quarto.org/docs/authoring/markdown-basics.html) and
-is a plain text file format (a type of file, like Word's `.docx`) that's designed to be easy to write and read.
+is a plain text file format (a type of file, like Word's `.docx`) that's
+designed to be easy to write and read.
 
 We won't cover how to write Markdown in this workshop. All you need to
 know is that we use Markdown files here because it is a common format
@@ -323,4 +324,11 @@ some of your ideas with your neighbour.
 
 ## Summary
 
--   TODO: List of summary items
+In this session, we have explored how to work with files on GitHub. In
+particular, we have:
+
+-   Added files and folders to a GitHub repository
+-   Renamed, edited, and moved files in the repository
+-   Deleted files and folders from the repository
+-   Explored the history of the repository to see the changes that have
+    been made