ZIP Exports: Finished up format doc, move files, started builder

Moved all existing export related app files into their new own dir.

app/Exports/Controllers/ChapterExportApiController.php

@@ -0,0 +1,66 @@
+<?php
+
+namespace BookStack\Exports\Controllers;
+
+use BookStack\Entities\Queries\ChapterQueries;
+use BookStack\Exports\ExportFormatter;
+use BookStack\Http\ApiController;
+use Throwable;
+
+class ChapterExportApiController extends ApiController
+{
+    public function __construct(
+        protected ExportFormatter $exportFormatter,
+        protected ChapterQueries $queries,
+    ) {
+        $this->middleware('can:content-export');
+    }
+
+    /**
+     * Export a chapter as a PDF file.
+     *
+     * @throws Throwable
+     */
+    public function exportPdf(int $id)
+    {
+        $chapter = $this->queries->findVisibleByIdOrFail($id);
+        $pdfContent = $this->exportFormatter->chapterToPdf($chapter);
+
+        return $this->download()->directly($pdfContent, $chapter->slug . '.pdf');
+    }
+
+    /**
+     * Export a chapter as a contained HTML file.
+     *
+     * @throws Throwable
+     */
+    public function exportHtml(int $id)
+    {
+        $chapter = $this->queries->findVisibleByIdOrFail($id);
+        $htmlContent = $this->exportFormatter->chapterToContainedHtml($chapter);
+
+        return $this->download()->directly($htmlContent, $chapter->slug . '.html');
+    }
+
+    /**
+     * Export a chapter as a plain text file.
+     */
+    public function exportPlainText(int $id)
+    {
+        $chapter = $this->queries->findVisibleByIdOrFail($id);
+        $textContent = $this->exportFormatter->chapterToPlainText($chapter);
+
+        return $this->download()->directly($textContent, $chapter->slug . '.txt');
+    }
+
+    /**
+     * Export a chapter as a markdown file.
+     */
+    public function exportMarkdown(int $id)
+    {
+        $chapter = $this->queries->findVisibleByIdOrFail($id);
+        $markdown = $this->exportFormatter->chapterToMarkdown($chapter);
+
+        return $this->download()->directly($markdown, $chapter->slug . '.md');
+    }
+}