Linen docs

Linen is an IDE for Minecraft mods and plugins: Fabric, Forge, NeoForge, Quilt, Legacy Fabric and Paper, from Minecraft 1.12.2 to 26.x.

• Getting started: make a mod, or open one you have, and run the game.
• Port a mod to 26.1: Yarn to Mojang names, Loom’s migrateMappings, mixins and NeoForge’s notes.
• Set up by loader: Fabric, Quilt and Legacy Fabric, NeoForge, Forge and Paper, with the Java, Gradle and plugin versions for each Minecraft version.
• Troubleshooting: mixin targets, Java and Gradle versions, sync failures, access wideners and transformers.

Linen is in private beta. These docs grow with it: mixins, working on several versions and more guides come next.