feat(builder): Add hierarchical subsection support for API reference sections

Implement support for multi-level section structure in API documentation,
allowing sections to contain subsections organized in folder hierarchies.

- Sections can now have subsections (e.g., FAQ.md + FAQ/ folder)
- Main section content displays as "Overview" subsection
- Subsection files automatically loaded from matching directories
- Dropdown navigation shows only subsection names, not parent section
- The new dynamic sections/subsections are bookmark-able the page
  automatically scrolls down to the section

Example structure:
  sections/Component/FAQ.md            "Overview" subsection
  sections/Component/FAQ/Example1.md   "Example1" subsection
  sections/Component/FAQ/Example2.md   "Example2" subsection

JIRA: BGSOFUIPIRIN-6925
Cherry-picked from UI5/openui5@05f6fef35.

Author: codeworrior

packages/builder/lib/processors/jsdoc/lib/transformApiJson.js

@@ -1034,15 +1034,41 @@ function transformer(sInputFile, sOutputFile, sLibraryFile, vDependencyAPIFiles,
 			const componentDir = path.join(sSectionsDir, sComponentPath);
 			if (fs.existsSync(componentDir)) {
 				try {
+				const dirContents = fs.readdirSync(componentDir, { withFileTypes: true });
+
+				// Separate files and directories in one pass
+				const mdFiles = [];
+				const dirNames = new Set();
+				dirContents.forEach((dirent) => {
+					if (dirent.isFile() && dirent.name.endsWith('.md')) {
+						mdFiles.push(dirent.name.replace(/\.md$/, ''));
+					} else if (dirent.isDirectory()) {
+						dirNames.add(dirent.name);
+					}
+				});
 
-					const customSections = fs.readdirSync(componentDir, { withFileTypes: true })
-						.filter((dirent) => dirent.isFile() && dirent.name.endsWith('.md'))
-						.map((dirent) => dirent.name.replace(/\.md$/, ''));
-
-					// Store list of available sections for this symbol
-					if (customSections.length > 0) {
-						symbol.customSections = customSections;
+				// Build customSections array with subsection support
+				const customSections = mdFiles.map(function(sectionName) {
+					// Check if there's a matching directory for this section
+					if (dirNames.has(sectionName)) {
+						try {
+							const subsectionFiles = fs.readdirSync(path.join(componentDir, sectionName), { withFileTypes: true })
+								.filter((dirent) => dirent.isFile() && dirent.name.endsWith('.md'))
+								.map((dirent) => dirent.name.replace(/\.md$/, ''));
+
+							if (subsectionFiles.length > 0) {
+								return { name: sectionName, hasSubsections: true, subsections: subsectionFiles };
+							}
+						} catch (error) {
+							log.error('Error scanning subsection directory:', path.join(componentDir, sectionName), error);
+						}
 					}
+					return sectionName;
+				});
+
+				if (customSections.length > 0) {
+					symbol.customSections = customSections;
+				}
 
 				} catch (error) {
 					log.error('Error scanning component sections directory:', componentDir, error);