Add instructions for linux setup

linux-start.sh

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+sudo apt-get install git curl wget p7zip-full
+
+sudo apt-get install git-lfs
+# for old system (ubuntu 16)
+#curl -s https://packagecloud.io/install/repositories/github/git-lfs/script.deb.sh | sudo bash
+#sudo apt-get install git-lfs
+
+# save login
+git config --global credential.helper store
+
+# clone build_tools
+git clone https://git.onlyoffice.com/ONLYOFFICE/build_tools.git
+
+# deps =========================================
+
+cd ./build_tools/tools/linux
+
+# python 3.10
+./python.sh
+
+# qt
+#./python3/bin/python3 ./qt_binary_fetch.py amd64
+#./python3/bin/python3 ./qt_binary_fetch.py arm64
+./python3/bin/python3 ./qt_binary_fetch.py all
+
+# deps
+./python3/bin/python3 ./deps.py
+
+# cmake 3.30
+sudo ./cmake.sh
+
+cd ../../
+
+# ==============================================
+
+# sysroots (IF NEEDED) =========================
+
+cd ./build_tools/tools/linux/sysroot
+#./python3/bin/python3 ./fetch.py amd64
+#./python3/bin/python3 ./fetch.py arm64
+./../python3/bin/python3 ./fetch.py all
+cd ../../../
+
+# ==============================================
+
+
+# configure ====================================
+
+./tools/linux/python3/bin/python3 ./configure.py --clean "0" --update-light "1" --update "1" --branch "hotfix/v9.2.1" --module "desktop" --qt-dir "$(pwd)/tools/linux/qt_build/Qt-5.9.9"
+
+# with sysroot: sysroot "1"
+
+# ==============================================
+
+# cross build linux_arm64
+sudo apt install qemu-user qemu-user-static binfmt-support
+sudo update-binfmts --enable qemu-aarch64
+
+# 1) without sysroot
+#sudo apt install gcc-aarch64-linux-gnu g++-aarch64-linux-gnu
+#sudo dpkg --add-architecture arm64
+#sudo apt update
+#... install all dev packages ...
+
+# 2) official supported: with sysroot
+./tools/linux/python3/bin/python3 ./configure.py sysroot "1" #...