docs: make yarn required in install guide

Since the move to Yarn workspaces in
10caf861e8, the package.json scripts rely
on Yarn being available to function properly due to the use of
workspaces.

Because of this, Yarn has basically been required to be able to properly
set up and run FoundKey. Using `npx yarn` probably won't work anymore
with modern yarn versions.

docs/INSTALL.md

@@ -11,6 +11,7 @@ FoundKey requires the following packages to run:
 * **[Node.js](https://nodejs.org/en/)** (18.x)
 * **[PostgreSQL](https://www.postgresql.org/)** (12.x minimum; 13.x+ is preferred)
 * **[Redis](https://redis.io/)**
+* **[Yarn](https://yarnpkg.com/)**
 
 The following are needed to compile native npm modules:
 * A C/C++ compiler like **GCC** or **Clang**
@@ -18,17 +19,16 @@ The following are needed to compile native npm modules:
 * **[Python](https://python.org/)** (3.x)
 
 ### Optional
-* [Yarn](https://yarnpkg.com/) - *If you decide not to install it, use `npx yarn` instead of `yarn`.*
 * [FFmpeg](https://www.ffmpeg.org/)
 
 To install the dependiencies on Debian (or derivatives like Ubuntu) you can use the following commands:
 ```sh
 curl -fsSL https://deb.nodesource.com/setup_18.x | bash -
 apt install build-essential python3 nodejs postgresql redis
+corepack enable # for yarn
 
 # Optional dependencies
 apt install ffmpeg
-corepack enable # for yarn
 ```
 
 ## Create FoundKey user