Slugs are broken in >caption blocks

[pepinho24]

Describe the bug
The content of the >caption blocks is converted manually to HTML, but the slugs are not normalized or resolved as it is done for the alert blocks, e.g. issues #14 and #212

Additional context
The slug parsing logic from the Alert generator plugin should be applied to the Caption generator plugin as well:

• https://github.com/telerik/docs-seed/blob/master/_plugins/alert_generator.rb#L23-L32
• https://github.com/telerik/docs-seed/blob/master/_plugins/caption_generator.rb#L16