Configuration validation

Validate the Kamal configuration giving useful warning on errors.
Each section of the configuration has its own config class and a YAML
file containing documented example configuration.

You can run `kamal docs` to see the example configuration, and
`kamal docs <section>` to see the example configuration for a specific
section.

The validation matches the configuration to the example configuration
checking that there are no unknown keys and that the values are of
matching types.

Where there is more complex validation - e.g for envs and servers, we
have custom validators that implement those rules.

Additonally the configuration examples are used to generate the
configuration documentation in the kamal-site repo.

You generate them by running:

```
bundle exec bin/docs <kamal-site-checkout>
```

lib/kamal/cli/healthcheck/poller.rb

@@ -6,7 +6,7 @@ module Kamal::Cli::Healthcheck::Poller
 
   def wait_for_healthy(pause_after_ready: false, &block)
     attempt = 1
-    max_attempts = KAMAL.config.healthcheck["max_attempts"]
+    max_attempts = KAMAL.config.healthcheck.max_attempts
 
     begin
       case status = block.call
@@ -33,7 +33,7 @@ module Kamal::Cli::Healthcheck::Poller
 
   def wait_for_unhealthy(pause_after_ready: false, &block)
     attempt = 1
-    max_attempts = KAMAL.config.healthcheck["max_attempts"]
+    max_attempts = KAMAL.config.healthcheck.max_attempts
 
     begin
       case status = block.call

lib/kamal/cli/main.rb

@@ -126,6 +126,18 @@ class Kamal::Cli::Main < Kamal::Cli::Base
     end
   end
 
+  desc "docs", "Show Kamal documentation for configuration setting"
+  def docs(section = nil)
+    case section
+    when NilClass
+      puts Kamal::Configuration.validation_doc
+    else
+      puts Kamal::Configuration.const_get(section.titlecase.to_sym).validation_doc
+    end
+  rescue NameError
+    puts "No documentation found for #{section}"
+  end
+
   desc "init", "Create config stub in config/deploy.yml and env stub in .env"
   option :bundle, type: :boolean, default: false, desc: "Add Kamal to the Gemfile and create a bin/kamal binstub"
   def init