Create search guide (opensearch-project#488)

Signed-off-by: Alexei Karikov <karikov.alist.ru@gmail.com>

guides/search.md

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+# Search
+OpenSearch provides a powerful search API that allows you to search for documents in an index. The search API supports a number of parameters that allow you to customize the search operation. In this guide, we will explore the search API and its parameters.
+
+# Setup
+Let's start by creating an index and adding some documents to it:
+
+```javascript
+const { Client } = require('@opensearch-project/opensearch');
+
+const client = new Client({
+  node: 'http://localhost:9200',
+});
+client.indices.create({index: 'movies'})
+
+
+for (let i = 0; i < 10; i++) {
+  client.index({
+    index: 'movies',
+    id: i,
+    body: {
+      title: `The Dark Knight ${i}`,
+      director: 'Christopher Nolan',
+      year: 2008 + i
+    }
+  });
+}
+
+client.index({
+  index: 'movies',
+  body: {
+    title: 'The Godfather',
+    director: 'Francis Ford Coppola',
+    year: 1972
+  }
+})
+client.index({
+  index: 'movies',
+  body: {
+    title: 'The Shawshank Redemption',
+    director: 'Frank Darabont',
+    year: 1994
+  }
+})
+
+client.indices.refresh({index: 'movies'}) // refresh the index to make the documents searchable
+```
+
+## Search API
+
+### Basic Search
+
+The search API allows you to search for documents in an index. The following example searches for ALL documents in the `movies` index:
+
+```javascript
+client.search({ index: 'movies'}).then(response => {
+  console.log(response.body.hits.total.value);
+})
+```
+
+You can also search for documents that match a specific query. The following example searches for documents that match the query `dark knight`:
+
+```javascript
+client.search({
+  index: 'movies',
+  body: {
+    query: {
+      match: {
+        title: 'dark knight'
+      }
+    }
+  }
+}).then(response => {
+  console.log(response.body.hits.hits);
+})
+```
+
+OpenSearch query DSL allows you to specify complex queries. Check out the [OpenSearch query DSL documentation](https://opensearch.org/docs/latest/query-dsl/) for more information.
+
+### Basic Pagination
+
+The search API allows you to paginate through the search results. The following example searches for documents that match the query `dark knight`, sorted by `year` in ascending order, and returns the first 2 results after skipping the first 5 results:
+
+```javascript
+search_body = {
+  query: {
+    match: {
+      title: 'dark knight'
+    }
+  },
+  sort: [
+    {
+      year: {
+        order: 'asc'
+      }
+    }
+  ]
+}
+
+client.search({
+  index: 'movies',
+  size: 2,
+  from: 5,
+  body: search_body
+}).then(response => {
+  console.log(response.body.hits.hits);
+})
+```
+
+With sorting, you can also use the `search_after` parameter to paginate through the search results. Let's say you have already displayed the first page of results, and you want to display the next page. You can use the `search_after` parameter to paginate through the search results. The following example will demonstrate how to get the first 3 pages of results using the search query of the previous example:
+
+```javascript
+page_1 = client.search({
+  index: 'movies',
+  size: 2,
+  body: search_body
+}).then(response => response.body.hits.hits)
+
+page2 = client.search({
+  index: 'movies',
+  size: 2,
+  body: {
+    ...search_body,
+    search_after: page1[page1.length - 1].sort
+  }
+}).then(response => response.body.hits.hits)
+
+page_3 = client.search({
+  index: 'movies',
+  size: 2,
+  body: {
+    ...search_body,
+    search_after: page_2[page2.length - 1].sort
+  }
+}).then(response => response.body.hits.hits)
+```
+
+### Pagination with scroll
+
+When retrieving large amounts of non-real-time data, you can use the `scroll` parameter to paginate through the search results. 
+
+```javascript
+page_1 = client.search({
+  index: 'movies',
+  scroll: '1m',
+  size: 2,
+  body: search_body
+})
+
+page_2 = client.scroll({
+  scroll_id: page_1._scroll_id,
+  scroll: '1m'
+})
+
+page_3 = client.scroll(
+  scroll_id: page_2._scroll_id,
+  scroll: '1m'
+)
+```
+
+### Pagination with Point in Time
+
+The scroll example above has one weakness: if the index is updated while you are scrolling through the results, they will be paginated inconsistently. To avoid this, you should use the "Point in Time" feature. The following example demonstrates how to use the `point_in_time` and `pit_id` parameters to paginate through the search results:
+
+```javascript
+// create a point in time
+const pit = client.create_pit({
+  index: 'movies',
+  keep_alive: '1m'
+});
+
+// Include pit info in the search body
+const pit_search_body = {
+  ...search_body,
+  pit: {
+    id: pit['pit_id'],
+    keep_alive: '1m'
+  }
+};
+
+// Get the first 3 pages of results
+const page_1 = client.search({
+  size: 2,
+  body: pit_search_body
+}).then(response => response.body.hits.hits)
+
+const page_2 = client.search({
+  size: 2,
+  body: {
+    ...pit_search_body,
+    search_after: page_1[page_1.length - 1].sort
+  }
+}).then(response => response.body.hits.hits)
+
+const page_3 = client.search({
+  size: 2,
+  body: {
+    ...pit_search_body,
+    search_after: page_2[page_2.length - 1].sort
+  }
+}).then(response => response.body.hits.hits)
+
+// Print out the titles of the first 3 pages of results
+console.log(page_1.map(hit => hit._source.title));
+console.log(page_2.map(hit => hit._source.title));
+console.log(page_3.map(hit => hit._source.title));
+
+// delete the point in time
+client.delete_pit({ body: { pit_id: pit.pit_id } });
+```
+Note that a point-in-time is associated with an index or a set of index. So, when performing a search with a point-in-time, you DO NOT specify the index in the search.
+
+## Cleanup
+
+```javascript
+client.indices.delete({index: 'movies'})
+```