getting-started: Bring toolbox up as peer, rework rpm-ostree

I think `toolbox` should be a clear peer of flatpak and rpm-ostree.
At some point I'd like to rework this page so we move `toolbox`
basics here, and demote `rpm-ostree` details to a separate page.

The rpm-ostree section is reworked to give clear examples of
packages to try layering and that need to be layered if desired.
And similarly give examples for `override replace`.  Let's not
emphasize `override remove` at all as its uses are obscure.

modules/ROOT/pages/getting-started.adoc

@@ -10,19 +10,16 @@ Silverblue has different options for installing software, compared with a
 standard Fedora Workstation (or other package-based Linux distributions). These 
 include:
 
-- Flatpak apps: this is the primary way that apps get installed on Silverblue.
-- Containers: which can be installed and used for development purposes.
-- Toolbox containers: a special kind of container that are tailored to be used 
-as a software development environment.
-
-The other method of installing software on Silverblue is package layering. 
-This is different from the other methods, and goes against the general 
-principle of immutability. Package layering adds individual packages to the 
-Silverblue system, and in so doing modifies the operating system. 
+- Flatpak apps: this is the primary way that (GUI) apps get installed on Silverblue.
+- Toolbox: Used primarily for CLI apps; development, debugging tools etc.
+- Package layering: The rpm-ostree tool used for host updates is a full hybrid
+  image/package system.  By default the system operates in pure image mode,
+  but package layering is useful for things like libvirt, drivers, etc.
 
 For information on link:flatpak[Flatpak] and link:package-layering[package 
-layering], see below. Information on containers can be found on the 
-link:toolbox[toolbox page].
+layering], see below.
+
+See the dedicated link:toolbox[toolbox page] to get started with it.
 
 [[flatpak]]
 == Flatpak
@@ -92,21 +89,21 @@ how to do this.
 == Package layering
 
 Package layering works by modifying your Silverblue installation. As the name 
-implies, it works by modifying the packages from which Silverblue is composed. 
-Any software package that is provided by Fedora can be installed on Silverblue 
-using this method.
+implies, it works by extending the packages from which Silverblue is composed. 
+
+Good examples of packages to be layered would be:
 
-A Silverblue installation with packages layered on top loses some of the 
-advantages of immutability: with layering in place, the system is no longer 
-identical to other installations, and is not as robust.
+ - `fish`: An alternative Unix shell
+ - `sway`: A Wayland tiling compositor
+ - `libvirt`: The libvirt daemon
 
-Nevertheless, package layering can be a useful option for installing 
-software that might not be available through other means, or for installing 
-software that, for technical reasons, needs to be part of the host operating 
-system.
+Most (but not all) RPM packages provided by Fedora can be installed on Silverblue 
+using this method.
 
-Typically, the system must be rebooted after a package has been layered. This 
-is an practical disadvantage compared with installing Flatpaks or containers.
+Currently, using package layering creates a new "deployment", or bootable filesystem
+root.  It does not affect your current root.  This preserves rollback and the transactional model, but means system must be rebooted after a package has been layered.
+Eventually this limitation may be lifted, but it's generally expected that you use package
+layering sparingly, and use `flatpak` and `dnf install` inside a `toolbox` etc.
 
 Package layering is generally done from the command line. However, the 
 Software application does rely on it for installing a small number of apps 
@@ -126,17 +123,15 @@ host, but can be used in a link:toolbox[toolbox]).
 Once a package has been installed in this manner, it will be kept up-to-date 
 as new versions are released and as the base operating system is updated.
 
-=== Removing and replacing packages
-
-A package can be removed from a Silverblue installation using:
+=== Replacing packages
 
-`$ rpm-ostree override remove <package name>`
-
-The same `rpm-ostree override` command can be used to replace a package with a 
-different version. For example:
+In some scenarios, you may want to e.g. test out a new version of `podman` or
+`kernel` or other packages that live on the host.  The `rpm-ostree override` command can be used to replace a package with a different version.  Currently you must download the RPM packages locally, and then run:
 
 `$ rpm-ostree override replace <path to package>`
 
+You may also use `override remove` to effectively "hide" packages; they will still exist in the underlying base layer, but will not appear in the booted root.
+
 Removing and replacing packages using package layering is not generally 
 recommended. For more information, see the 
 https://rpm-ostree.readthedocs.io/en/latest/manual/administrator-handbook/[rpm-ostree documentation].