Add the hcl label to Terraform snippets (gravitational#30055)

The docs engine now supports syntax highlighting for HCL snippets. This
change enables HCL syntax highlighting for some Terraform guides in the
docs.

docs/pages/access-controls/login-rules/terraform.mdx

@@ -78,7 +78,7 @@ provider and create two example Login Rules.
 Make sure to update the `addr = "teleport.example.com:443"` field with the
 public address of your Teleport Proxy.
 
-```
+```hcl
 (!examples/resources/terraform/terraform-login-rules.tf!)
 ```
 

docs/pages/agents/deploy-agents-terraform.mdx

@@ -102,7 +102,7 @@ pool in your infrastructure.
 
 The file `agent-pool.tf` configures EC2 instances and Teleport join tokens:
 
-```text
+```hcl
 (!examples/agent-pool-terraform/agent-pool.tf!)
 ```
 
@@ -150,14 +150,14 @@ configuration.
 The Terraform configuration we show in this guide relies on the following inputs
 (`examples/agent-pool-terraform/inputs.tf`):
 
-```text
+```hcl
 (!examples/agent-pool-terraform/inputs.tf!)
 ```
 
 In your `agent-pool-terraform` project directory, create a file called
 `main.auto.tfvars` with the following content:
 
-```text
+```hcl
 agent_count=2
 proxy_service_address="mytenant.teleport.sh"
 aws_region=""
@@ -187,7 +187,7 @@ Finally, make sure you are using the latest supported version of the Teleport
 Terraform provider. The `required_providers` block for the Teleport provider
 includes a placeholder value:
 
-```text
+```hcl
 (!examples/agent-pool-terraform/provider.tf!)
 ```
 
@@ -246,7 +246,7 @@ reference](../reference/terraform-provider.mdx).
 <Tabs>
 <TabItem label="Application">
 
-```text
+```hcl
 resource "teleport_app" "example" {
   metadata = {
     name        = "example"
@@ -267,7 +267,7 @@ resource "teleport_app" "example" {
 </TabItem>
 <TabItem label="Database">
 
-```text
+```hcl
 resource "teleport_database" "example" {
   metadata = {
     name        = "example"

docs/pages/management/dynamic-resources/terraform-provider.mdx

@@ -144,12 +144,12 @@ To prepare a Terraform configuration file:
 
    <Tabs>
    <TabItem scope={["cloud","team"]} label="Cloud-Hosted">
-   ```
+   ```hcl
    (!examples/resources/terraform/terraform-user-role-cloud.tf!)
    ```
    </TabItem>
    <TabItem scope={["oss", "enterprise"]} label="Self-Hosted">
-   ```
+   ```hcl
    (!examples/resources/terraform/terraform-user-role-self-hosted.tf!)
    ```
    </TabItem>