docs: add conditional expressions

This update adds a new section explaining conditional expressions in HCL, including syntax, usage examples, and best practices for clarity and maintainability. The documentation covers how to use conditionals for default values, dynamic argument omission, and recommends using locals for complex logic.

Ref: #12806

Signed-off-by: Ryan Johnson <ryan@tenthirtyam.org>

website/content/docs/templates/hcl_templates/expressions.mdx

@@ -1,8 +1,7 @@
 ---
 page_title: Expressions - Configuration Language
 description: |-
-  HCL allows the use of expressions to access data exported
-  by sources and to transform and combine that data to produce other values.
+  HCL expressions provide access to data exported by data sources and transforms and combines the data into other values. Learn how to use HCL expressions in Packer templates.
 ---
 
 # Expressions
@@ -142,6 +141,106 @@ The following named values are available:
 For a full list of available functions, see [the function
 reference](/packer/docs/templates/hcl_templates/functions).
 
+## Conditional Expressions
+
+A conditional expression uses the value of a boolean expression to select one of
+two values. This is a compact way to express `if-then-else` logic inside an
+expression.
+
+The syntax for a conditional expression is:
+
+```hcl
+condition ? true_val : false_val
+```
+
+If `condition` is `true`, the expression returns `true_val`. If `condition` is
+`false`, it returns `false_val`. The `true_val` and `false_val` arguments must
+be of compatible types.
+
+### Examples
+
+Here are some examples of how to use conditional expressions.
+
+1.  Setting a Default Value
+
+    You can use a conditional to provide a default value for a variable that
+    can also be customized.
+
+    ```hcl
+    locals {
+      # Set the region to the value of var.region if it is not empty.
+      # Otherwise, use a default value.
+      region = var.region != "" ? var.region : "us-east-1"
+    }
+    ```
+
+2.  Dynamically Omitting an Argument
+
+    You can use a `null` value to dynamically omit an argument from a source.
+    This is a common pattern to effectively "turn off" or prevent an argument
+    from being set.
+
+    ```hcl
+    variable "use_http" {
+      type        = bool
+      description = "Whether to use HTTP for file transfer."
+      default     = true
+    }
+
+    source "example" "foo" {
+      # The http_directory argument is only set if var.use_http is true.
+      # If false, http_directory will be unset.
+      http_directory = var.use_http ? "/path/to/files" : null
+    }
+    ```
+
+    This can also be used to set one of two mutually exclusive arguments:
+
+    ```hcl
+    source "builder" "example" {
+      # If var.use_http is true, http_content is set and cd_content is null.
+      # If var.use_http is false, http_content is null and cd_content is set.
+      http_content = var.use_http ? local.http_files : null
+      cd_content   = !var.use_http ? local.cd_files : null
+    }
+    ```
+
+### Readability and Recommended Practices
+
+While powerful, conditional expressions can lead to complex or hard-to-read
+configurations if not used carefully.
+
+  * **Prioritize Clarity:** When a conditional expression becomes very long or
+    involves multiple nested conditions, consider breaking it down.
+
+  * **Leverage `locals` for Complexity:** For more intricate conditional logic,
+    define the result in a `local` variable. This improves readability in the
+    main `source` or `provisioner` blocks where the `local` is consumed.
+
+    **Example:**
+
+    ```hcl
+    locals {
+      # Determine the kickstart file based on the build environment.
+      kickstart_config_path = var.environment == "production" ?
+                              "config/prod-ks.cfg" :
+                              (var.environment == "staging" ?
+                               "config/stage-ks.cfg" :
+                               "config/dev-ks.cfg")
+    }
+
+    source "builder" "example" {
+      # ...
+      # Use the pre-determined kickstart path.
+      http_content = file(local.kickstart_config_path)
+      # ...
+    }
+    ```
+
+  * **Use `null` to Omit Arguments:** As demonstrated in the examples,
+    using `null` is the idiomatic way to prevent an argument from being set or
+    to effectively "turn off" a setting based on a condition.
+
 ## `for` Expressions
 
 A _`for` expression_ creates a complex type value by transforming