Added .NET client API guide for the Search API. (opensearch-project#394)

Fix setup section in guide.



Minor updates to search.md



Added requested changes to search.md



Added requested changes to search.md #2



Added requested changes to search.md #3

Signed-off-by: Djcarrillo6 <[email protected]>

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:
+
+```csharp
+using OpenSearch.Client;
+using OpenSearch.Net;
+
+var node = new Uri("https://localhost:9200");
+var config = new ConnectionSettings(node)
+    .ThrowExceptions()
+    .ServerCertificateValidationCallback(CertificateValidations.AllowAll)
+    .BasicAuthentication("admin", "admin");
+var client = new OpenSearchClient(config);
+
+class Movie
+{
+    public string Title { get; set; }
+    public string Director { get; set; }
+    public int Year { get; set; }
+    public override string ToString()
+    {
+        return $"{nameof(Title)}: {Title}, {nameof(Director)}: {Director}, {nameof(Year)}: {Year}";
+    }
+}
+
+await client.Indices.CreateAsync("movies", c => c
+    .Settings(s => s
+        .NumberOfShards(1)
+        .NumberOfReplicas(0)
+    )
+    .Map<Movie>(m => m
+        .Properties(p => p
+            .Text(t => t
+                .Name(o => o.Title))
+            .Text(t => t
+                .Name(o => o.Director))
+            .Number(n => n
+                .Name(o => o.Year)
+                .Type(NumberType.Integer)))));
+
+var movies = new List<Movie>
+{
+    new Movie { Title = "The Godfather", Director = "Francis Ford Coppola", Year = 1972 },
+    new Movie { Title = "The Shawshank Redemption", Director = "Frank Darabont", Year = 1994 },
+};
+
+for (var i = 0; i < 10; ++i)
+    movies.Add(new Movie { Title = "The Dark Knight " + i, Director = "Christopher Nolan", Year = 2008 + i });
+
+// Index the movies
+var indexResponse = await client.BulkAsync(b => b
+    .Index("movies")
+    .IndexMany(movies)
+    .Refresh(Refresh.WaitFor));
+Console.WriteLine($"Indexed {indexResponse.Items.Count} movies"); // Indexed 12 movies
+```
+
+
+## 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:
+
+```csharp
+var searchResponse = await client.SearchAsync<Movie>(s => s
+    .Index("movies")
+    .Query(q => q.MatchAll()));
+Console.WriteLine(string.Join('\n', searchResponse.Documents));
+```
+
+You can also search for documents that match a specific query. The following example searches for documents that match the query `dark knight`:
+```csharp
+var searchResponse = await client.SearchAsync<Movie>(s => s
+    .Index("movies")
+    .Query(q => q
+        .Match(m => m
+            .Field(f => f.Title)
+            .Query("dark knight"))));
+Console.WriteLine(string.Join('\n', searchResponse.Documents));
+```
+
+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 in ascending order, and returns the first 2 results after skipping the first 5 results:
+
+```csharp
+var query = Query<Movie>.Match(m => m
+    .Field(f => f.Title)
+    .Query("dark knight"));
+var sort = new SortDescriptor<Movie>()
+    .Ascending(f => f.Year);
+var searchResponse = await client.SearchAsync<Movie>(s => s
+    .Index("movies")
+    .Query(_ => query)
+    .Sort(_ => sort)
+    .From(5)
+    .Size(2));
+Console.WriteLine(string.Join('\n', searchResponse.Documents));
+```
+
+With sorting, you can also use the `SearchAfter` 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 `SearchAfter` 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:
+
+```csharp
+var page1 = await client.SearchAsync<Movie>(s => s
+    .Index("movies")
+    .Query(_ => query)
+    .Sort(_ => sort)
+    .Size(2));
+var page2 = await client.SearchAsync<Movie>(s => s
+    .Index("movies")
+    .Query(_ => query)
+    .Sort(_ => sort)
+    .Size(2)
+    .SearchAfter(page1.Hits.Last().Sorts));
+var page3 = await client.SearchAsync<Movie>(s => s
+    .Index("movies")
+    .Query(_ => query)
+    .Sort(_ => sort)
+    .Size(2)
+    .SearchAfter(page2.Hits.Last().Sorts));
+Console.WriteLine(string.Join('\n', page3.Documents));
+```
+
+### Pagination with Scroll API
+When retrieving large amounts of non-real-time data, you can use the `scroll` parameter to paginate through the search results:
+
+```csharp
+var page1 = await client.SearchAsync<Movie>(s => s
+    .Index("movies")
+    .Query(_ => query)
+    .Sort(_ => sort)
+    .Size(2)
+    .Scroll("1m"));
+var page2 = await client.ScrollAsync<Movie>("1m", page1.ScrollId);
+var page3 = await client.ScrollAsync<Movie>("1m", page2.ScrollId);
+Console.WriteLine(string.Join('\n', page3.Documents));
+```
+
+
+## Cleanup
+```csharp
+await client.Indices.DeleteAsync("movies");
+```