TD-1419 Use DocumentComponent subclasses everywhere

[adjam]

Previously, we were only using a subclass of Blacklight::DocumentComponent on the "index" (results) page; this PR adds a new subclass TrlnArgon::DocumentShowComponent for use on the "show" (full record) page.

The new "show" subclass adds a sub_header slot for the basic Imprint/ resource type/ format subheader; the sample app/views/catalog/_show_main_content.html.erb template shows how to add this subheader to the output.

To support this change:

• app/views/catalog/_show_main_content.html.rb has been reworked to use the new component class; since we are all overriding this template anyway (based on a quick Slack survey 2025-02-20), in order to use the component we'll have to edit our overrides. This version of th` e template is thus more of a "how to" or basic starting point. Downstream applications that do not change their version of this template will not be using the new component class to render the full record page.
• renamed TrlnArgon::DocumentComponent to TrlnArgon::DocumentIndexComponent to harmonize its naming with the new sublclass.
• added configuration in lib/trln_argon/controller_override.rb to use the new components.

Additionally, added a default value for the TrlnArgon::StartOverButtonComponent#initialize method; the default template for this component did not set classes: and this caused errors when using the component with the default template.