License CC:T according to the REUSE specification (#1351)

This adds SPDX license headers to all source code files, following the
REUSE[1] specification. This does not include any asset files (such as
generated JSON files, or textures). While REUSE does support doing so
with ".license" files, for now we define these licences using the
.reuse/dep5 file.

[1]: https://reuse.software/

doc/guides/gps_setup.md

@@ -2,6 +2,12 @@
 module: [kind=guide] gps_setup
 ---
 
+<!--
+SPDX-FileCopyrightText: 2022 The CC: Tweaked Developers
+
+SPDX-License-Identifier: LicenseRef-CCPL
+-->
+
 # Setting up GPS
 The @{gps} API allows computers and turtles to find their current position using wireless modems.
 

doc/guides/local_ips.md

@@ -2,6 +2,12 @@
 module: [kind=guide] local_ips
 ---
 
+<!--
+SPDX-FileCopyrightText: 2022 The CC: Tweaked Developers
+
+SPDX-License-Identifier: MPL-2.0
+-->
+
 # Allowing access to local IPs
 By default, ComputerCraft blocks access to local IP addresses for security. This means you can't normally access any
 HTTP server running on your computer. However, this may be useful for testing programs without having a remote

doc/guides/speaker_audio.md

@@ -4,6 +4,12 @@ see: speaker.playAudio Play PCM audio using a speaker.
 see: cc.audio.dfpwm Provides utilities for encoding and decoding DFPWM files.
 ---
 
+<!--
+SPDX-FileCopyrightText: 2021 The CC: Tweaked Developers
+
+SPDX-License-Identifier: MPL-2.0
+-->
+
 # Playing audio with speakers
 CC: Tweaked's speaker peripheral provides a powerful way to play any audio you like with the @{speaker.playAudio}
 method. However, for people unfamiliar with digital audio, it's not the most intuitive thing to use. This guide provides

doc/guides/using_require.md

@@ -2,6 +2,12 @@
 module: [kind=guide] using_require
 ---
 
+<!--
+SPDX-FileCopyrightText: 2021 The CC: Tweaked Developers
+
+SPDX-License-Identifier: MPL-2.0
+-->
+
 # Reusing code with require
 A library is a collection of useful functions and other definitions which is stored separately to your main program. You
 might want to create a library because you have some functions which are used in multiple programs, or just to split