Issue #3335371 by beeyayjay,msielski: Make visual display showing entity references.

Combining existing entity_diagram module into content_model_documentation.

Requires installation of https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.min.js into site libraries/mermaid directory. Would like to automate this in the future or allow choice between local or CDN (Webform has a nice example of this). At minimum we still need to detect if this library is present.

Re-namespaced things best I could - feel free to update class names, URLs or anything you see fit.

Diagram is at /admin/reports/entity_diagram and discoverable via Reports.

I did not get to create settings where I had discussed. @beeyayjay indicated she'd like to work on that and it could be its own MR I think. These ares are marked as TODOs in the code. All I did was remove the hard-coded entity types from the code as they broke things, and rolled back to some older code which queries Drupal for available, fieldable entities.

css/diagram.css

+/* Stylesheet for Content Model Documentation Entity Diagram */
+
+#content-model-documentation-diagram-form {
+  display: flex;
+  max-width: 1000px;
+  border: #ccc solid 1px;
+  padding: 8px;
+  margin-bottom: 20px;
+}
+
+#content-model-documentation-diagram-form .form-item,
+#content-model-documentation-diagram-form #bundles {
+  flex: auto;
+  margin: 0 12px;
+}
+
+#content-model-documentation-diagram-form #submit_button {
+  margin: 10px 12px;
+}
+
+/* Default mermaid elements to hidden, then show with JS to prevent FOUC. */
+.mermaid {
+  opacity: 0;
+}