Fix broken anchors

Exhaustive list of all broken anchors found:
- Broken anchor on source page path = /docs/about/humhub:
   -> linking to /docs/professional-edition/features##feature-list
- Broken anchor on source page path = /docs/about/releasenotes/release_notes_1_5:
   -> linking to /docs/develop/modules-migrate#migrate-from-14-to-15
- Broken anchor on source page path = /docs/about/releasenotes/release_notes_1_7:
   -> linking to /docs/admin/uploads#compression--downscaling
- Broken anchor on source page path = /docs/admin/logging:
   -> linking to /docs/admin/security#security-configuration
- Broken anchor on source page path = /docs/develop/content:
   -> linking to /docs/develop/modules-base-class#use-of-contentcontainermodule
   -> linking to /docs/develop/modules#use-of-container-module
- Broken anchor on source page path = /docs/develop/environment:
   -> linking to /docs/develop/build#build-assets
- Broken anchor on source page path = /docs/develop/modules:
   -> linking to /docs/develop/modules-base-class#use-of-contentcontainermodule
- Broken anchor on source page path = /docs/develop/modules-base-class:
   -> linking to #container-module (resolved as: /docs/develop/modules-base-class#container-module)
- Broken anchor on source page path = /docs/develop/modules-event-handler:
   -> linking to /docs/develop/overview#configphp
   -> linking to #activerecord-events (resolved as: /docs/develop/modules-event-handler#activerecord-events)
- Broken anchor on source page path = /docs/develop/modules-lifecycle:
   -> linking to /docs/develop/environment#external-modules-directory
   -> linking to /docs/develop/modules-base-class#use-of-contentcontainermodule
- Broken anchor on source page path = /docs/develop/modules-migrate:
   -> linking to /docs/develop/files#replacemodify
- Broken anchor on source page path = /docs/theme/migrate:
   -> linking to /docs/admin/advanced-configuration#module-configuration
- Broken anchor on source page path = /docs/user/spaces:
   -> linking to /docs/adminuser/adminusers#Groups