feat: improve the image management experience with template builder (#27018)

Makes it easier to pick the right workspace image, both in the template
builder and in the docs.

- Template builder: the Docker and Kubernetes bases now expose a
`container_image` variable in the wizard (freeform text, defaults to
`codercom/example-base:ubuntu`), and their prerequisites explain why
image choice matters, with tradeoffs between
`codercom/example-base:ubuntu` (minimal) and
`codercom/example-universal:ubuntu` (catch-all), plus pointers to
[coder/images](https://github.com/coder/images) and the image management
docs.
- Docs: reworked [image
management](https://coder.com/docs/@ben%2Fdevrel-201-image-guidance-prereqs/admin/templates/managing-templates/image-management)
into a clearer maturity ladder (minimal → golden → project-specific →
developer customization), with pullable image references in every
example, `codercom/oss-dogfood` as a project-specific example, and Dev
Containers + [mise](https://mise.jdx.dev/) as ways to customize without
new images.

Companion PR for the starter templates: coder/registry#943

Part of DEVREL-201.

🤖 Generated with Coder Agents using Claude, on behalf of @bpmct (wizard
variable by @jeremyruppel in #27024)

---------

Co-authored-by: Jeremy Ruppel <jeremyruppel@users.noreply.github.com>

coderd/templatebuilder_handler_test.go

@@ -45,13 +45,14 @@ func TestTemplateBuilderBases(t *testing.T) {
 			{
 				id:           "docker",
 				expectedOS:   "linux",
-				hasVariables: false,
+				hasVariables: true,
+				expectedVars: []string{"container_image"},
 			},
 			{
 				id:           "kubernetes",
 				expectedOS:   "linux",
 				hasVariables: true,
-				expectedVars: []string{"namespace", "use_kubeconfig"},
+				expectedVars: []string{"container_image", "namespace", "use_kubeconfig"},
 			},
 			{
 				id:           "aws-linux",