Updated files in docs

Started to fill in missing information in the docs so we can size down readme file as mentioned #2485

edited usage.rst (added flashing and exporting with pmbootstrap, QEMU, Device Porting Assistance) added debugging.rst (with all content from readme) added chroot.rst (with all content from readme and an example)

hope this goes into the wanted direction...

docs/chroot.rst

+######
+Chroot
+######
+
+chroot (short for "change root") is a system call and command that changes the apparent root directory.
+This can come in very handy to make some modifications in the generated image file.
+
+
+
+Enter the armhf building chroot:
+
+.. code-block:: shell
+
+  $ pmbootstrap chroot -b armhf
+
+
+Run a command inside a chroot:
+
+.. code-block:: shell
+
+  $ pmbootstrap chroot -- echo test
+
+
+Safely delete all chroots:
+
+.. code-block:: shell
+
+  $ pmbootstrap zap
+
+
+
+Use ``chroot`` to install a specific kernel version from an apk file. 
+
+
+copy your working kernel apk to the chroot dir of pmbootstrap:
+
+.. code-block:: shell
+
+  $ sudo cp /path/of/linux-kernel.apk ~/.local/var/pmbootstrap/chroot_rootfs_oneplus-enchilada/ 
+
+enter chroot
+
+.. code-block:: shell
+
+  $ pmbootstrap chroot -r 
+
+
+and install the package:
+
+.. code-block:: shell
+
+ $ apk add linux-kernel.apk
+
+