Simplify Migration Class Version Syntax

Updated the syntax for defining migration versions from `self.version =` to a simpler and more concise `Cql::Migration(version)` format. This change promotes cleaner code and easier readability. Additionally, removed redundant schema definition examples from documentation to prevent confusion and enhance clarity.

docs/Cql/Migration.md

@@ -17,9 +17,7 @@ The `Migrator` class also provides methods to list applied and pending migration
 **Example** Creating a new migration
 
 ```crystal
-class CreateUsersTable < Cql::Migration
-  self.version = 1_i64
-
+class CreateUsersTable < Cql::Migration(1)
   def up
     schema.alter :users do
       add_column :name, String

docs/Cql/Migrator.md

@@ -12,14 +12,6 @@ The `Migrator` class also provides methods to list applied and pending migration
 **Example** Creating a new migrator
 
 ```crystal
-schema = Cql::Schema.define(:northwind, "sqlite3://db.sqlite3") do |s|
-  table :schema_migrations do
-    primary :id, Int32
-    column :name, String
-    column :version, Int64, index: true, unique: true
-    timestamps
-  end
-end
 migrator = Cql::Migrator.new(schema)
 ```
 

docs/coreconcepts/migrations.md

@@ -17,9 +17,7 @@ Migrations allow you to:
 Let’s start with a simple example. Suppose we need to add a `users` table to our database with two columns: `name` and `age`.
 
 ```crystal
-class CreateUsersTable < Cql::Migration
-  self.version = 1_i64
-
+class CreateUsersTable < Cql::Migration(1)
   def up
     schema.alter :users do
       add_column :name, String
@@ -158,9 +156,7 @@ This gives you details about the last migration that was successfully applied.
 Here’s an example where we define multiple migrations and apply them sequentially:
 
 ```crystal
-class CreateMoviesTable < Cql::Migration
-  self.version = 2_i64
-
+class CreateMoviesTable < Cql::Migration(2)
   def up
     schema.alter :movies do
       add_column :title, String

src/migrations.cr

@@ -13,9 +13,7 @@ module Cql
   # **Example** Creating a new migration
   #
   # ```
-  # class CreateUsersTable < Cql::Migration
-  #   self.version = 1_i64
-  #
+  # class CreateUsersTable < Cql::Migration(1)
   #   def up
   #     schema.alter :users do
   #       add_column :name, String
@@ -35,14 +33,6 @@ module Cql
   # **Example** Applying migrations
   #
   # ```
-  # schema = Cql::Schema.define(:northwind, "sqlite3://db.sqlite3") do |s|
-  #   table :schema_migrations do
-  #     primary :id, Int32
-  #     column :name, String
-  #     column :version, Int64, index: true, unique: true
-  #     timestamps
-  #   end
-  # end
   # migrator = Cql::Migrator.new(schema)
   # migrator.up
   # ```