Downloading the instructions (#1160)

## What's Changed?

- Includes instructions in the project download if there are any
- Adds a list of reserved file names which includes `INSTRUCTIONS.md` to
prevent clashes

Closes
RaspberryPiFoundation/digital-editor-issues#374

CHANGELOG.md

@@ -18,6 +18,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Show instructions option in sidebar if instructions are editable (#1164)
 - Open instructions panel by default if instructions are editable (#1164)
 - Instructions empty state to show when instructions are editable (#1165)
+- Downloading project instructions (#1160)
 
 ### Changed