It would be great to have the possibility of preview each section using main theme instead of the admin one as in cannonical entity route & showing rendered "layout_section" as it would do when imported in layout builder content design page.
Go to "Content" -> "Layout builder library" -> (Choose one) "Operations links" -> "Preview": "/admin/content/section-library" -> (example) "/admin/content/section-library/32/preview".
Or configure Label in view "/admin/structure/views/view/section_library" with rewrite results as link with "/admin/content/section-library/{{ id }}/preview" & do same as described but click in each section label.
Add a new controller to facilitate that feature & include as entity preview link.
Confirm approach or iterate like proposed below.
Editors with "view section library templates" access (same as required to enter the main sections view) will be able to preview sections without having to include it into content just to check how it would look like (like manual screenshot but "live", updated, automatically & large enough, screenshot may be shown in low dimensions within import from library).
None for initial version using "entity_id" to use it as context (required in many cases to build sections correctly).
That approach has a disvantage as if entity where section was saved is removed, that context can't be used, using section otherwhise but may not show some blocks that requires specific entity like node body block.
A sample entity can be generated by "\Drupal::service('layout_builder.sample_entity_generator')->get($entity_type_id, $entity_bundle_id)," but those paramenters are missing in entity data, also consider that for sections saved from display configuration "https://drupal.org/i/3263496" stored entity type is "entity_view_display".
An iteration maybe would be to store entity type id & bundle of entity where section is saved, or section storage id?
Other complementary option would be to replace cannonical like media does if a configuration is enabled "standalone" or by default as display seems not configurable by ui & sections are not shown.
Needs review
1.1
Code
First commented approach ready to review, please read issue description, thanks!