feat: lay the site out as a reading funnel

Five tabs from Start here to Contribute, a level on every page, a device picker on Get your files that lists every registered platform, and platform pages that open on where the files go, the command, the pack and how to check. FAQ and troubleshooting keep the questions asked before installing, the technical ones move to their own page, and getting started points to the pages that took its content.

wiki/advanced-usage.md

@@ -1,7 +1,26 @@
-# Advanced usage
+---
+level: reference
+---
+
+# Build your own packs
 
 Fine-grained control over pack generation, hardware filtering, truth analysis, and verification.
 
+The [latest release](https://github.com/Abdess/retrobios/releases/latest)
+carries one pack for each platform, and that pack holds everything the
+platform runs: the BIOS list it publishes, plus every file its emulator cores
+load, read from their source code. There is no lighter variant to choose,
+because a pack that leaves out a file a core needs means a game that does not
+boot, with no message saying why. It is not a guarantee either: source
+profiles can document files nobody has dumped or that only the user can
+provide, all of them visible in the [gap analysis](../gaps.md).
+
+From a clone of the repository, `python scripts/generate_pack.py --platform
+retroarch --region us` builds a narrower pack, `--required-only` the bare
+minimum each core needs to start, and `--no-game-data` keeps the firmware and
+leaves out the files a game reads. `--help` lists every way to build your own,
+and the sections below describe each one.
+
 ## Custom packs
 
 ### Build from hash