Clean up Javadocs a little

I've no motivation for modding right now, but always got time for build
system busywork!

CC:T (and CC before that) has always published its API docs. However,
they're not always the most helpful — they're useful if you know what
you're looking for, but aren't a good getting-started guide.

Part of the issue here is there's no examples, and everything is
described pretty abstractly. I have occasionally tried to improve this
(e.g. the peripheral docs in bdffabc08e),
but it's a long road.

This commit adds a new example mod, which registers peripherals, an API
and a turtle upgrade. While the mod itself isn't exported as part of the
docs, we reference blocks of it using Java's new {@snippet} tag.

 - Switch the Forge project to use NeoForge's new Legacy MDG plugin. We
   don't *need* to do this, but it means the build logic for Forge and
   NeoForge is more closely aligned.

 - Add a new SnippetTaglet, which is a partial backport of Java 18+'s
   {@snippet}.

 - Add an example mod. This is a working multi-loader mod, complete with
   datagen (albeit with no good multi-loader abstractions).

 - Move our existing <pre>{@code ...}</pre> blocks into the example mod,
   replacing them with {@snippet}s.

 - Add a new overview page to the docs, providing some getting-started
   information. We had this already in the dan200.computercraft.api
   package docs, but it's not especially visible there.

config/checkstyle/checkstyle.xml

@@ -124,7 +124,7 @@ SPDX-License-Identifier: MPL-2.0
         </module>
         <module name="MethodTypeParameterName" />
         <module name="PackageName">
-            <property name="format" value="^(dan200\.computercraft|cc\.tweaked)(\.[a-z][a-z0-9]*)*" />
+            <property name="format" value="^(dan200\.computercraft|cc\.tweaked|com\.example\.examplemod)(\.[a-z][a-z0-9]*)*" />
         </module>
         <module name="ParameterName" />
         <module name="StaticVariableName">