Issue #2798531 by quietone, jp.stacey, ranjith_kumar_k_u, Ankit.Gupta,...

 * @{

 * Information about the classes and interfaces that make up the Block API.

 *

 * @section sec_overview Overview

 * Blocks are a combination of a configuration entity and a plugin. The

 * configuration entity stores placement information (theme, region, weight) and

 * any other configuration that is specific to the block. The block plugin does

 * the work of rendering the block's content for display.

 *

 * @section sec_requirements Basic requirements

 * To define a block in a module you need to:

 * - Define a Block plugin by creating a new class that implements the

 *   \Drupal\Core\Block\BlockPluginInterface, in namespace Plugin\Block under your

 *   @link annotation Annotations topic @endlink for more information about

 *   annotations.

 *

 * This is an example of a basic block plugin class:

 * @code

 * namespace Drupal\my_module\Plugin\Block;

 *

 * use Drupal\Core\Block\BlockBase;

 * #[Block(

 *   id: "my_block",

 *   admin_label: new TranslatableMarkup("My Block"),

 * )]

 * class MyBlock extends BlockBase {

 *   public function build() {

 *     return [

 *       '#type' => '#markup',

 *       '#markup' => 'Example block',

 *     ];

 *   }

 * }

 * @endcode

 *

 * More examples are available at the links below.

 *

 * @section sec_extending Extending blocks with conditions and hooks

 * The Block API also makes use of Condition plugins, for conditional block

 * placement. Condition plugins have interface

 * \Drupal\Core\Condition\ConditionInterface, base class

 * - hook_block_view_BASE_BLOCK_ID_alter()

 * - hook_block_access()

 *

 * Further information and examples:

 * @section sec_further_information Further information

 * - \Drupal\system\Plugin\Block\SystemPoweredByBlock provides a simple example

 *   of defining a block.

 * - \Drupal\user\Plugin\Condition\UserRole is a straightforward example of a

 * - \Drupal\system\Plugin\Block\SystemMenuBlock is an example of a block with

 *   a custom configuration form.

 * - For a more in-depth discussion of the Block API, see

 *   https://www.drupal.org/developing/api/8/block_api.

 *   https://www.drupal.org/docs/drupal-apis/block-api/block-api-overview.

 * - The Examples for Developers project also provides a Block example in

 *   https://www.drupal.org/project/examples.

 * @}