[documentation] It is not clear which commands are described

https://github.com/hashicorp/consul-template#commands

I do not understand which commands are described and how to use them.

Particularly, I want to fetch value from aws secret manager: `aws secretsmanager get-secret-value --secret-id ${PROJECT_NAME}/path --region ${AWS_REGION}`. Is next possible?

```
job{ group { task {
template {
data = <<-EOH
{{- with exec "aws secretmanager ..." -}}
PGPASSWORD="{{ .Data.password }}"
PGUSER="{{ .Data.user }}"
PGDATABASE="{{ .Data.db }}"
{{ end }}

EOH
destination = "secrets/file.env"
env = true
}}}
```

Start with the commands section linked in the issue and review the documented command syntax against the AWS Secrets Manager example. Clarify which commands are available, how they are invoked, and whether the shown use case is supported; done means a newcomer can identify the relevant command and follow an accurate example.

Written by the indexing model from the issue text.