mdbook: miscellaneous cleanup

* Remove an unnecessary index in SUMMARY.md
* Fix section levels in usage.md

BUG=b:195003973
TEST=mdbook build

Change-Id: Icf97038f664d02d169f9db30f4999f053ed2ed81
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3188673
Commit-Queue: Dennis Kempin <denniskempin@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Dennis Kempin <denniskempin@google.com>
diff --git a/docs/book/src/SUMMARY.md b/docs/book/src/SUMMARY.md
index 20f146f..1c1c995 100644
--- a/docs/book/src/SUMMARY.md
+++ b/docs/book/src/SUMMARY.md
@@ -13,10 +13,10 @@
 
 --------------------------------------------------------------------------------
 
+
 -   [Appendix](./appendix/index.md)
     -   [Architectural Overview](./appendix/architectural_overview.md)
     -   [Example Usage (Outdated)](./appendix/example_usage.md)
-    -   [Features](./appendix/features.md)
     -   [Sandboxing](./appendix/sandboxing.md)
     -   [Seccomp](./appendix/seccomp.md)
     -   [Minijail](./appendix/minijail.md)
diff --git a/docs/book/src/running_crosvm/usage.md b/docs/book/src/running_crosvm/usage.md
index d107b59..64230f5 100644
--- a/docs/book/src/running_crosvm/usage.md
+++ b/docs/book/src/running_crosvm/usage.md
@@ -1,9 +1,9 @@
-## Usage
+# Usage
 
 To see the usage information for your version of crosvm, run `crosvm` or `crosvm
 run --help`.
 
-### Boot a Kernel
+## Boot a Kernel
 
 To run a very basic VM with just a kernel and default devices:
 
@@ -14,9 +14,9 @@
 The uncompressed kernel image, also known as vmlinux, can be found in your
 kernel build directory in the case of x86 at `arch/x86/boot/compressed/vmlinux`.
 
-### Rootfs
+## Rootfs
 
-#### With a disk image
+### With a disk image
 
 In most cases, you will want to give the VM a virtual block device to use as a
 root file system:
@@ -44,7 +44,7 @@
 > **NOTE:** If more disks arguments are added prior to the desired rootfs image,
 > the `root=/dev/vda` must be adjusted to the appropriate letter.
 
-#### With virtiofs
+### With virtiofs
 
 Linux kernel 5.4+ is required for using virtiofs. This is convenient for
 testing. The file system must be named "mtd*" or "ubi*".
@@ -54,7 +54,7 @@
     -p "rootfstype=virtiofs root=mtdfake" vmlinux
 ```
 
-### Control Socket
+## Control Socket
 
 If the control socket was enabled with `-s`, the main process can be controlled
 while crosvm is running. To tell crosvm to stop and exit, for example:
@@ -74,7 +74,7 @@
 allowing it to clean up any OS resources that might have stuck around if crosvm
 were terminated early.
 
-### Multiprocess Mode
+## Multiprocess Mode
 
 By default crosvm runs in multiprocess mode. Each device that supports running
 inside of a sandbox will run in a jailed child process of crosvm. The
@@ -83,7 +83,7 @@
 `--seccomp-policy-dir` argument. The sandbox can be disabled for testing with
 the `--disable-sandbox` option.
 
-### Virtio Wayland
+## Virtio Wayland
 
 Virtio Wayland support requires special support on the part of the guest and as
 such is unlikely to work out of the box unless you are using a Chrome OS kernel
@@ -93,7 +93,7 @@
 the path `$XDG_RUNTIME_DIR/wayland-0` points to the socket of the Wayland
 compositor you would like the guest to use.
 
-### GDB Support
+## GDB Support
 
 crosvm supports [GDB Remote Serial Protocol] to allow developers to debug guest
 kernel via GDB.