docs: psvita: add install/build instructions

Documentation/psvita.md

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+## PlayStation Vita port
+
+### Prerequisites
+1. Make sure your PSVita is [set up to run homebrew applications](https://vita.hacks.guide/).
+2. Install [kubridge](https://github.com/TheOfficialFloW/kubridge/releases/) by copying `kubridge.suprx` to your taiHEN plugins folder (usually `ux0:/tai`) and add it to your `config.txt`, for example:
+   ```
+   *KERNEL
+   ux0:tai/kubridge.skprx
+   ```
+3. Install `libshacccg.suprx` by following [this guide](https://cimmerian.gitbook.io/vita-troubleshooting-guide/shader-compiler/extract-libshacccg.suprx).
+
+### Installation
+1. If you have an old vitaXash3D install, remove it.
+2. Get `xash3d-fwgs-psvita.7z` from the latest [automatic build](https://github.com/FWGS/xash3d-fwgs/releases/tag/continuous).
+3. Install `xash.vpk` from the 7z archive onto your PSVita.
+4. Copy the `data` directory from the 7z archive to the root of your PSVita's SD card.
+5. Copy the valve folder and any other mod folders from your Half-Life install to `ux0:/data/xash3d/` (you can use other mountpoints instead of `ux0`). **Do not overwrite anything.**
+
+### Build instructions
+1. Install [VitaSDK](https://vitasdk.org/).
+2. Build and install [vitaGL](https://github.com/Rinnegatamante/vitaGL):
+    ```
+    git clone https://github.com/Rinnegatamante/vitaGL.git
+    make -C vitaGL NO_TEX_COMBINER=1 HAVE_UNFLIPPED_FBOS=1 HAVE_PTHREAD=1 SINGLE_THREADED_GC=1 MATH_SPEEDHACK=1 DRAW_SPEEDHACK=1 HAVE_CUSTOM_HEAP=1 -j2 install
+    ```
+3. Build and install [vita-rtld](https://github.com/fgsfdsfgs/vita-rtld):
+    ```
+    git clone https://github.com/fgsfdsfgs/vita-rtld.git && cd vita-rtld
+    mkdir build && cd build
+    cmake -DCMAKE_BUILD_TYPE=Release ..
+    make -j2 install
+    ```
+4. Build and install [this SDL2 fork](https://github.com/Northfear/SDL) with vitaGL integration:
+    ```
+    git clone https://github.com/Northfear/SDL.git && cd SDL
+    mkdir build && cd build
+    cmake -DCMAKE_TOOLCHAIN_FILE=${VITASDK}/share/vita.toolchain.cmake -DCMAKE_BUILD_TYPE=Release -DVIDEO_VITA_VGL=ON ..
+    make -j2 install
+    ```
+5. Use `waf`:
+    ```
+    ./waf configure -T release --psvita
+    ./waf build
+    ```
+6. Copy all the resulting `.so` files into a single folder:
+    ```
+    ./waf install --destdir=xash3d
+    ```
+7. `xash.vpk` is located in `build/engine/`.