Remove old instructions & patches they referred to

(& rename instructions to the actual state)

Old instruction names were from the time when tdesktop used GYP that had MSVC, XCode and CMake backends.
But tdesktop uses CMake now, so these names are no longer correct.

docs/building-linux.md

@@ -0,0 +1,44 @@
+## Build instructions for Linux using Docker
+
+### Obtain your API credentials
+
+You will require **api_id** and **api_hash** to access the Telegram API servers. To learn how to obtain them [click here][api_credentials].
+
+### Clone source code
+
+    git clone --recursive https://github.com/telegramdesktop/tdesktop.git
+
+### Prepare libraries
+
+Go to the `tdesktop` directory and run
+
+    docker build -t tdesktop:centos_env Telegram/build/docker/centos_env/
+
+### Building the project
+
+Make sure that you're still in the `tdesktop` directory and run (using [your **api_id** and **api_hash**](#obtain-your-api-credentials))
+
+    docker run --rm -it \
+        -v $PWD:/usr/src/tdesktop \
+        tdesktop:centos_env \
+        /usr/src/tdesktop/Telegram/build/docker/centos_env/build.sh \
+        -D TDESKTOP_API_ID=YOUR_API_ID \
+        -D TDESKTOP_API_HASH=YOUR_API_HASH \
+        -D DESKTOP_APP_USE_PACKAGED=OFF \
+        -D DESKTOP_APP_DISABLE_CRASH_REPORTS=OFF
+
+Or, to create a debug build, run (also using [your **api_id** and **api_hash**](#obtain-your-api-credentials))
+
+    docker run --rm -it \
+        -v $PWD:/usr/src/tdesktop \
+        -e DEBUG=1 \
+        tdesktop:centos_env \
+        /usr/src/tdesktop/Telegram/build/docker/centos_env/build.sh \
+        -D TDESKTOP_API_ID=YOUR_API_ID \
+        -D TDESKTOP_API_HASH=YOUR_API_HASH \
+        -D DESKTOP_APP_USE_PACKAGED=OFF \
+        -D DESKTOP_APP_DISABLE_CRASH_REPORTS=OFF
+
+The built files will be in the `out` directory.
+
+[api_credentials]: api_credentials.md