docs and changelog for `nomad config validate` (#12031)

.changelog/9198.txt

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+```release-note:improvement
+cli: add `nomad config validate` command to check configuration files without an agent
+```

website/content/docs/commands/config/index.mdx

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+---
+layout: docs
+page_title: 'Commands: config'
+description: |
+  The config command interacting with configurations.
+---
+
+# Command: config
+
+The `config` command is used to interact with configurations.
+
+## Usage
+
+Usage: `nomad config <subcommand> [options]`
+
+Run `nomad config <subcommand> -h` for help on that subcommand. The
+following subcommands are available:
+
+- [`config validate`][validate] - Validate configuration files
+
+[validate]: /docs/commands/config/validate 'Validate configuration files'

website/content/docs/commands/config/validate.mdx

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+---
+layout: docs
+page_title: 'Commands: config validate'
+description: |
+  The config validate command is used to validate Nomad configuration files.
+---
+
+# Command: config validate
+
+The `config validate` command performs validation on a set of Nomad
+configuration files. This is useful to test the Nomad configuration
+without starting the agent.
+
+## Usage
+
+```plaintext
+nomad config validate <config_path> [<config_path...>]
+```
+
+The `config validate` command requires a path to either a single
+configuration file or a directory of configuration files to use for
+configuring the Nomad agent. This option can be specified multiple
+times. If multiple config files are passed, the values from each will
+be merged together. During merging, values from files found later in
+the list are merged over values from previously parsed files.
+
+This command cannot operate on partial configuration fragments since
+those won't pass the full agent validation. This command does not
+require an ACL token.
+
+Returns 0 if the configuration is valid, or 1 if there are problems.
+
+## General Options
+
+@include 'general_options.mdx'
+
+## Examples
+
+Validate a configuration file:
+
+```shell-session
+$ nomad config validate ./nomad.hcl
+Configuration is valid!
+```
+
+Validate a directory of configuration files:
+
+```shell-session
+$ nomad config validate /etc/nomad.d
+Configuration is valid!
+```

website/data/docs-nav-data.json

@@ -315,6 +315,19 @@
           }
         ]
       },
+      {
+        "title": "config",
+        "routes": [
+          {
+            "title": "Overview",
+            "path": "commands/config"
+          },
+          {
+            "title": "validate",
+            "path": "commands/config/validate"
+          }
+        ]
+      },
       {
         "title": "deployment",
         "routes": [