add legacy_isotime docs (#10928)

website/content/docs/templates/hcl_templates/functions/datetime/index.mdx

@@ -1,4 +0,0 @@
----
-page_title: datetime - Functions - Configuration Language
-description: Overview of available datetime functions
----

website/content/docs/templates/hcl_templates/functions/datetime/legacy_isotime.mdx

@@ -0,0 +1,41 @@
+---
+page_title: legacy_isotime - Functions - Configuration Language
+description: |-
+  The legacy_isotime function returns a string representation of the current date
+  and time.
+---
+
+# `legacy_isotime` Function
+
+The `legacy_isotime` function returns the current date and time using the given format string.
+The format string follows golang's datetime formatting. See
+[isotime-function-format-reference](/docs/templates/legacy_json_templates/engine#isotime-function-format-reference)
+for more details.
+
+This function has been provided to create backwards compatibility with
+Packer's legacy JSON templates. However, we recommend that you upgrade your
+HCL Packer template to use [`timestamp`](/docs/templates/hcl_templates/functions/datetime/timestamp)
+and [`formatdate`](/docs/templates/hcl_templates/functions/datetime/formatdate) together as soon as is convenient.
+
+-> **Note:** If you are using a large number of builders, provisioners
+or post-processors, the isotime may be slightly different for each one
+because it is from when the plugin is launched not the initial Packer
+process. In order to avoid this and make the timestamp consistent across all
+plugins, set it as a user variable and then access the user variable within
+your plugins.
+
+## Examples
+
+```shell-session
+> legacy_isotime("2006-01-02")
+2021-04-19
+```
+
+## Related Functions
+
+- [`format`](/docs/templates/hcl_templates/functions/string/format) is a more general formatting function for arbitrary
+data.
+- [`timestamp`](/docs/templates/hcl_templates/functions/datetime/timestamp) returns the current date and time in a format
+suitable for input to `formatdate`.
+- [`formatdate`](/docs/templates/hcl_templates/functions/datetime/formatdate) can convert the resulting timestamp to
+  other date and time formats.

website/data/docs-nav-data.json

@@ -463,10 +463,6 @@
               {
                 "title": "Date and Time Functions",
                 "routes": [
-                  {
-                    "title": "Overview",
-                    "path": "templates/hcl_templates/functions/datetime"
-                  },
                   {
                     "title": "formatdate",
                     "path": "templates/hcl_templates/functions/datetime/formatdate"
@@ -478,6 +474,10 @@
                   {
                     "title": "timestamp",
                     "path": "templates/hcl_templates/functions/datetime/timestamp"
+                  },
+                  {
+                    "title": "legacy_isotime",
+                    "path": "templates/hcl_templates/functions/datetime/legacy_isotime"
                   }
                 ]
               },