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

tests/Api/SearchApiTest.php

@@ -0,0 +1,47 @@
+<?php
+
+namespace Tests\Api;
+
+use BookStack\Entities\Models\Book;
+use BookStack\Entities\Models\Bookshelf;
+use BookStack\Entities\Models\Chapter;
+use BookStack\Entities\Models\Entity;
+use BookStack\Entities\Models\Page;
+use Tests\TestCase;
+
+class SearchApiTest extends TestCase
+{
+    use TestsApi;
+
+    protected $baseEndpoint = '/api/search';
+
+    public function test_all_endpoint_returns_search_filtered_results_with_query()
+    {
+        $this->actingAsApiEditor();
+        $uniqueTerm = 'MySuperUniqueTermForSearching';
+
+        /** @var Entity $entityClass */
+        foreach ([Page::class, Chapter::class, Book::class, Bookshelf::class] as $entityClass) {
+            /** @var Entity $first */
+            $first = $entityClass::query()->first();
+            $first->update(['name' => $uniqueTerm]);
+            $first->indexForSearch();
+        }
+
+        $resp = $this->getJson($this->baseEndpoint . '?query=' . $uniqueTerm . '&count=5&page=1');
+        $resp->assertJsonCount(4, 'data');
+        $resp->assertJsonFragment(['name' => $uniqueTerm, 'type' => 'book']);
+        $resp->assertJsonFragment(['name' => $uniqueTerm, 'type' => 'chapter']);
+        $resp->assertJsonFragment(['name' => $uniqueTerm, 'type' => 'page']);
+        $resp->assertJsonFragment(['name' => $uniqueTerm, 'type' => 'bookshelf']);
+    }
+
+    public function test_all_endpoint_requires_query_parameter()
+    {
+        $resp = $this->actingAsApiEditor()->get($this->baseEndpoint);
+        $resp->assertStatus(422);
+
+        $resp = $this->actingAsApiEditor()->get($this->baseEndpoint . '?query=myqueryvalue');
+        $resp->assertOk();
+    }
+}