merge AppImage builder into main repo with revised linux guide (#78)

Tested as working.

It is self-contained with no dependencies, uses relative paths, downloads what it needs on the fly, requires the main repo to already be cloned and built for native linux, so it doesn't need to be a separate repo.

- Copied the `build.sh` file and `assets` folder into a new `AppImageBuilder` folder on the torzu repo root.
- Created a `AppImage-build.sh` shortcut on the repo root that checks if an executable is already in `build/bin`. If it doesn't find one, it prompts the user to build a native version first. If it does find one:
    - enters the `AppImageBuilder` folder
    - runs the command `./build.sh ../build ./torzu.AppImage`, using the correct relative path to the build folder (if they followed the build instructions as directed)
    - moves the resulting `torzu.AppImage` executable back into the main `torzu` folder
    - returns back to the main folder and shows current directory contents
- Added relevant entries to `.gitignore`
- Added AppImage section to linux build guide, with some clarifying explanation for Flatpaks, and section separators

Co-authored-by: anon <anon@no.reply>
Reviewed-on: http://vub63vv26q6v27xzv2dtcd25xumubshogm67yrpaz2rculqxs7jlfqad.onion/torzu-emu/torzu/pulls/78
Co-authored-by: anon <anon@noreply.localhost>
Co-committed-by: anon <anon@noreply.localhost>

AppImageBuilder/assets/bwrap-info.txt

@@ -0,0 +1 @@
+This is a statically compiled bubblewrap 0.9.0 executable.

AppImageBuilder/assets/torzu.desktop

@@ -0,0 +1,6 @@
+[Desktop Entry]
+Type=Application
+Name=torzu
+Icon=torzu
+Exec=AppRun
+Categories=Game;

AppImageBuilder/assets/torzu.svg

@@ -0,0 +1,2 @@
+<?xml version="1.0" encoding="UTF-8"?>
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