[docs] refactor: full rewrite, generalization + dedup (#488)

"docs but awesome"

Combines most of the stuff that was repeated thrice over verbatim into a
single common Build Instructions page, with additional caveats marked
elsewhere. Prettifies some stuff too because why not.

cc: @Lizzie @DraVee @MaranBr @SDK-Chan

Co-authored-by: Caio Oliveira <caiooliveirafarias0@gmail.com>
Reviewed-on: #488
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: MaranBr <maranbr@eden-emu.dev>

docs/scripts/Linux.md

@@ -0,0 +1,31 @@
+# Linux Build Scripts
+
+* Provided script: `.ci/linux/build.sh`
+* Must specify arch target, e.g.: `.ci/linux/build.sh amd64`
+* Valid targets:
+  * `native`: Optimize to your native host architecture
+  * `legacy`: x86\_64 generic, only needed for CPUs older than 2013 or so
+  * `amd64`: x86\_64-v3, for CPUs newer than 2013 or so
+  * `steamdeck` / `zen2`: For Steam Deck or Zen >= 2 AMD CPUs (untested on Intel)
+  * `rog-ally` / `allyx` / `zen4`: For ROG Ally X or Zen >= 4 AMD CPUs (untested on Intel)
+  * `aarch64`: For armv8-a CPUs, older than mid-2021 or so
+  * `armv9`: For armv9-a CPUs, newer than mid-2021 or so
+* Extra CMake flags go after the arch target.
+
+### Environment Variables
+
+* `NPROC`: Number of compilation threads (default: all cores)
+* `TARGET`: Set `appimage` to disable standalone `eden-cli` and `eden-room`
+* `BUILD_TYPE`: Build type (default: `Release`)
+
+Boolean flags (set `true` to enable, `false` to disable):
+
+* `DEVEL` (default `FALSE`): Disable Qt update checker
+* `USE_WEBENGINE` (default `FALSE`): Enable Qt WebEngine
+* `USE_MULTIMEDIA` (default `FALSE`): Enable Qt Multimedia
+
+* AppImage packaging script: `.ci/linux/package.sh`
+
+  * Accepts same arch targets as build script
+  * Use `DEVEL=true` to rename app to `Eden Nightly`
+  * This should generally not be used unless in a tailor-made packaging environment (e.g. Actions/CI)