Improve Paragraphs preprocess, THEME_preprocess_paragraph, field + twig documentation

Problem/Motivation

I would like to create a Paragraph instance with two features using theme preprocessing - and since the D8 documentation is scattered on the issue it would help to pull it together. Here on this thread, we can pull together some disparate parts which would end up on the documentation page.

I would like to get the image file source URL out of an image field inside a paragraph, turn that into a twig variable using THEME_preprocess_paragraph. Additionally another select list field would let me add a class to help determine the positioning of a background image. And the element with the class would have twig markup similar to, for example:

<div class="{{backgroundposition}}" style="background-image: url({{imagelocation}})"></div>

So using THEME_preprocess_paragraph (or another hook, like the paragraph's field hook if needed) how can I collect the value of a select list field from the paragraph, and the URL from the image field (assuming it is [0] and not a repeater image field)?

Additional questions are in which templates are various paragraph twig variables available? How best to explore and debug them?

Relevant theming hook in paragraphs.module:

/**
 * Implements hook_theme_suggestions_HOOK().
 */
function paragraphs_theme_suggestions_paragraph(array $variables) {
  $suggestions = array();
  $paragraph = $variables['elements']['#paragraph'];
  $sanitized_view_mode = strtr($variables['elements']['#view_mode'], '.', '_');

  $suggestions[] = 'paragraph__' . $sanitized_view_mode;
  $suggestions[] = 'paragraph__' . $paragraph->bundle();
  $suggestions[] = 'paragraph__' . $paragraph->bundle() . '__' . $sanitized_view_mode;

  return $suggestions;
}

Remaining tasks

• Edit page here: https://www.drupal.org/node/2444893

See also + related bug threads:

• #2797101: Can I extend template_preprocess_paragraph function?
• #2758411: Render paragraph.bundle as attribute in template (Layout using Display Suite) This worked: {{ 'paragraph--type--' ~ content['#bundle']|clean_class }} {{ 'paragraph--view-mode--' ~ content['#view_mode']|clean_class }} )
• #2678104: Get image URI from paragraph Similar to what I am looking for: "the best way to do this is using a preprocess hook to add it as a separate variable that is then exposed in twig"
• https://www.drupal.org/docs/8/theming-drupal-8/modifying-attributes-in-a...
• #2799927: Unable to access nested paragraphs in paragraph_preprocess function has a hook_preprocess_field strategy.
• #2769543: Issues with preprocess function Has snippet for field data on THEME_preprocess_paragraph.
• #2662766: Render another Paragraph field value inside field--paragraph--field....html.twig Working with multiple values in a field.
• #2662400: Getting a value of a field for an if statement in D8 Another THEME_preprocess_paragraph snippet.

Here is a useful snippet: ( via #2769543: Issues with preprocess function)

function base_preprocess_paragraph(&$variables) {
	$paragraph = $variables['paragraph'];
	$variables['heading'] = $paragraph->field_heading->get(0)->value;
}

For debugging,

function THEME_preprocess_paragraph__text_large_image(&$variables) {
  $paragraph = $variables['paragraph'];
dpm($paragraph);

I will share working snippets here as I move ahead on this, with an eye to writing a better documentation page from it all.