Add documentation page for encrypted tenant attributes (#174)

* wip docs page related to `archtechx/tenancy/issues/760`

* Update encrypt-attributes.blade.md

* Update the docs page

* Minor update

* Update the page

* Update first attribute encryption paragraph

* Improve wording

Co-authored-by: Samuel Štancl <samuel@archte.ch>

* Update hyperlink

* Change 'tenants migration' to 'tenants table'

* Move Tenant attribute encryption in the navigation

* Remove virtual column hyperlink

Co-authored-by: lukinovec <lukinovec@gmail.com>
Co-authored-by: Samuel Štancl <samuel@archte.ch>

source/docs/v3/tenant-attribute-encryption.blade.md

@@ -0,0 +1,65 @@
+---
+title: Tenant attribute encryption
+extends: _layouts.documentation
+section: content
+---
+
+# Tenant attribute encryption {#encrypt}
+
+To encrypt attributes on the Tenant model, store them in [custom columns]({{ $page->link('tenants/#custom-columns') }}) and cast the attributes to `'encrypted'`, or your custom encryption cast.
+
+For example, we'll encrypt the tenant's database credentials – `tenancy_db_username` and `tenancy_db_password`. We need to create custom columns for these attributes, because by default, they are stored in the virtual `data` column.
+
+- Add custom columns to the tenants table (we recommend making the string size at least 512 characters, so the string is capable of containing the encrypted data):
+
+```php
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+class CreateTenantsTable extends Migration
+{
+    /**
+     * Run the migrations.
+     *
+     * @return void
+     */
+    public function up(): void
+    {
+        Schema::create('tenants', function (Blueprint $table) {
+            $table->string('id')->primary();
+
+            // Your custom columns
+            $table->string('tenancy_db_username', 512);
+            $table->string('tenancy_db_password', 512);
+
+            $table->timestamps();
+            $table->json('data')->nullable();
+        });
+    }
+}
+```
+
+- Define the custom columns on the Tenant model:
+
+```php
+public static function getCustomColumns(): array
+{
+    return [
+        'id',
+        'tenancy_db_username',
+        'tenancy_db_password',
+    ];
+}
+```
+
+- Then define casts for the attributes on the model (using [Laravel's encrypted casts](https://laravel.com/docs/9.x/eloquent-mutators#encrypted-casting), or your custom casts):
+
+```php
+protected $casts = [
+    'tenancy_db_username' => 'encrypted',
+    'tenancy_db_password' => 'encrypted',
+];
+```