Add an example showing how the content of a block is changed at runtime

We need to use merge requests instead of patches.

Given the comments in that class, the empty array is returned on purpose, to show that empty blocks are not added to pages.

  /**
   * {@inheritdoc}
   *
   * The return value of the build() method is a renderable array. Returning an
   * empty array will result in empty block contents. The front end will not
   * display empty blocks.
   */

    // We return an empty array on purpose. The block will thus not be rendered
    // on the site. See BlockExampleTest::testBlockExampleBasic().

An example showing how the content of a block is changed at runtime can be added, but it needs to alter a different block.