config-linux: Consistent header nesting

[wking]

We have an explicit policy for example header levels, so follow that in config-linux.md. Also adjust the non-example headers to be one level deeper than their parent.

Spun off from here, and we've had similar changes before in #650. After this commit:

$ grep '^#' config-linux.md
# <a name="linuxContainerConfiguration" />Linux Container Configuration
## <a name="configLinuxDefaultFilesystems" />Default Filesystems
## <a name="configLinuxNamespaces" />Namespaces
### Example
## <a name="configLinuxUserNamespaceMappings" />User namespace mappings
### Example
## <a name="configLinuxDevices" />Devices
### Example
### <a name="configLinuxDefaultDevices" />Default Devices
## <a name="configLinuxControlGroups" />Control groups
### Example
### <a name="configLinuxDeviceWhitelist" />Device whitelist
#### Example
### <a name="configLinuxDisableOutOfMemoryKiller" />Disable out-of-memory killer
#### Example
### <a name="configLinuxMemory" />Memory
#### Example
### <a name="configLinuxCPU" />CPU
#### Example
### <a name="configLinuxBlockIO" />Block IO
#### Example
### <a name="configLinuxHugePageLimits" />Huge page limits
#### Example
### <a name="configLinuxNetwork" />Network
#### Example
### <a name="configLinuxPIDS" />PIDs
#### Example
## <a name="configLinuxIntelRdt" />IntelRdt
### Example
## <a name="configLinuxSysctl" />Sysctl
### Example
## <a name="configLinuxSeccomp" />Seccomp
### Example
## <a name="configLinuxRootfsMountPropagation" />Rootfs Mount Propagation
### Example
## <a name="configLinuxMaskedPaths" />Masked Paths
### Example
## <a name="configLinuxReadonlyPaths" />Readonly Paths
### Example
## <a name="configLinuxMountLabel" />Mount Label
### Example

[hqhq]

LGTM

[mrunalp]

LGTM