Added API search endpoint

Is a little awkward, emulates a 'list' API endpoint but has unstable
paging and does not support filters/sort. This is detailed on the
endpoint though.

Made some updates to the docs system to better support parameters
and examples on GET requests.

Includes tests to cover.

For #909

app/Http/Controllers/Api/SearchApiController.php

@@ -0,0 +1,67 @@
+<?php
+
+namespace BookStack\Http\Controllers\Api;
+
+use BookStack\Entities\Models\Entity;
+use BookStack\Entities\Tools\SearchOptions;
+use BookStack\Entities\Tools\SearchRunner;
+use Illuminate\Http\Request;
+
+class SearchApiController extends ApiController
+{
+    protected $searchRunner;
+
+    protected $rules = [
+        'all' => [
+            'query' => ['required'],
+            'page'  => ['integer', 'min:1'],
+            'count'  => ['integer', 'min:1', 'max:100'],
+        ],
+    ];
+
+    public function __construct(SearchRunner $searchRunner)
+    {
+        $this->searchRunner = $searchRunner;
+    }
+
+    /**
+     * Run a search query against all main content types (shelves, books, chapters & pages)
+     * in the system. Takes the same input as the main search bar within the BookStack
+     * interface as a 'query' parameter. See https://www.bookstackapp.com/docs/user/searching/
+     * for a full list of search term options. Results contain a 'type' property to distinguish
+     * between: bookshelf, book, chapter & page.
+     *
+     * The paging parameters and response format emulates a standard listing endpoint
+     * but standard sorting and filtering cannot be done on this endpoint. If a count value
+     * is provided this will only be taken as a suggestion. The results in the response
+     * may currently be up to 4x this value.
+     */
+    public function all(Request $request)
+    {
+        $this->validate($request, $this->rules['all']);
+
+        $options = SearchOptions::fromString($request->get('query') ?? '');
+        $page = intval($request->get('page', '0')) ?: 1;
+        $count = min(intval($request->get('count', '0')) ?: 20, 100);
+
+        $results = $this->searchRunner->searchEntities($options, 'all', $page, $count);
+
+        /** @var Entity $result */
+        foreach ($results['results'] as $result) {
+            $result->setVisible([
+                'id', 'name', 'slug', 'book_id',
+                'chapter_id', 'draft', 'template',
+                'created_at', 'updated_at',
+                'tags', 'type',
+            ]);
+            $result->setAttribute('type', $result->getType());
+        }
+
+        return response()->json([
+            'data' => $results['results'],
+            'total' => $results['total'],
+        ]);
+    }
+
+
+}