mdbook: Add page of "Basic Usage"

Add a starter document.
The most of content is moved from example usage doc [1] except GUI features.

[1]: https://google.github.io/crosvm/appendix/example_usage.html

BUG=b:195003973
TEST=none

Change-Id: I5fd1398795087edc1b41984cb579b59b9f34be5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3188677
Tested-by: kokoro <noreply+kokoro@google.com>
Commit-Queue: Keiichi Watanabe <keiichiw@chromium.org>
Reviewed-by: Daniel Verkamp <dverkamp@chromium.org>

docs/book/src/SUMMARY.md

@@ -5,7 +5,8 @@
     -   [For Linux](./building_crosvm/linux.md)
     -   [For Chromium OS](./building_crosvm/chromium_os.md)
 -   [Running Crosvm](./running_crosvm/index.md)
-    -   [Usage](./running_crosvm/usage.md)
+    -   [Basic Usage](./running_crosvm/basic_usage.md)
+    -   [Command Line Options](./running_crosvm/usage.md)
     -   [System Requirements](./running_crosvm/requirements.md)
     -   [Features](./running_crosvm/features.md)
     -   [Devices](./running_crosvm/devices.md)

docs/book/src/running_crosvm/basic_usage.md

@@ -0,0 +1,87 @@
+# Basic Usage
+
+This page describes how to prepare VM artifacts and run crosvm with them. If you
+haven't built crosvm yet, please read the [Building crosvm] section first.
+
+[Building crosvm]: ../building_crosvm/index.md
+
+## Get a Kernel
+
+Because crosvm usually runs the latest stable Chrome OS kernel, that is the most
+convenient one to use. If you are using the chroot for Chromium OS development,
+you already have the kernel source. Otherwise, you can clone it:
+
+```bash
+git clone --depth 1 -b chromeos-5.10 https://chromium.googlesource.com/chromiumos/third_party/kernel
+```
+
+Either way that you get the kernel, the next steps are to configure and build
+the bzImage:
+
+```bash
+make chromiumos-container-vm-x86_64_defconfig
+make -j$(nproc) bzImage
+```
+
+This kernel does not build any modules, nor does it support loading them, so
+there is no need to worry about an initramfs, although they are supported in
+crosvm.
+
+## Get a rootfs disk
+
+This stage enjoys the most flexibility. There aren't any special requirements
+for a rootfs in crosvm, but you will at a minimum need an init binary. This
+could even be `/bin/bash` if that is enough for your purposes. To get you
+started, a Debian rootfs can be created with [debootstrap]. Make sure to define
+`$CHROOT_PATH`.
+
+```bash
+truncate -s 20G debian.ext4
+mkfs.ext4 debian.ext4
+mkdir -p "${CHROOT_PATH}"
+sudo mount debian.ext4 "${CHROOT_PATH}"
+sudo debootstrap stable "${CHROOT_PATH}" http://deb.debian.org/debian/
+sudo chroot "${CHROOT_PATH}"
+passwd
+echo "tmpfs /tmp tmpfs defaults 0 0" >> /etc/fstab
+echo "tmpfs /var/log tmpfs defaults 0 0" >> /etc/fstab
+echo "tmpfs /root tmpfs defaults 0 0" >> /etc/fstab
+echo "sysfs /sys sysfs defaults 0 0" >> /etc/fstab
+echo "proc /proc proc defaults 0 0" >> /etc/fstab
+exit
+sudo umount "${CHROOT_PATH}"
+```
+
+[debootstrap]: https://wiki.debian.org/Debootstrap
+
+> Note: If you run crosvm on a testing device (e.g. Chromebook in Developer
+> mode),　another option is to share the host's rootfs with the guest via
+> virtiofs. See the [virtiofs usage](./usage.html#with-virtiofs).
+
+<!-- TODO: Is it possible to use a prebuilt Debian image downloaded from the official site? -->
+
+## Create a disk image
+
+You can simply create a disk image as follows:
+
+```bash
+fallocate --length 4G disk.img
+mkfs.ext4 ./disk.img
+```
+
+## Launch a VM
+
+Now, you can start a VM with the `crosvm run` command. Note that the user must
+be added to a group which can access `/dev/kvm` (usually `kvm`).
+
+```bash
+./target/debug/crosvm run \
+  --disable-sandbox \
+  --rwroot /path/to/debian.ext4 \
+  --rwdisk /path/to/disk.img \
+  -p 'init=/bin/bash' \
+  /path/to/kernel/vmlinux
+```
+
+Once the guest started, disk.img will be available as `/dev/vdb` in the guest.
+For more advanced usage, see the next section.