HelpSection plugins now can have weight for order

Modules can add sections to the admin/help page by defining a HelpSection plugin. See
https://api.drupal.org/api/drupal/core%21modules%21help%21src%21HelpSect...
for more information about HelpSection plugins.

These plugins can now provide a weight property in their annotation, in order to define their position on the admin/help page. Existing core sections are given the following weights:
- Help topics (experimental module): -10
- hook_help() module overviews: 0
- Tours: 10

Example annotation with weight, for a HelpSection plugin:

 * @HelpSection(
 *   id = "help_topics",
 *   title = @Translation("Topics"),
 *   weight = -10,
 *   description = @Translation("Topics can be provided by modules or themes. Top-level help topics on your site:"),
 *   permission = "access administration pages"
 * )