[Backport 2.18] add keyword analyzer docs

_analyzers/keyword.md

@@ -0,0 +1,77 @@
+---
+layout: default
+title: Keyword analyzer
+nav_order: 80
+---
+
+# Keyword analyzer
+
+The `keyword` analyzer doesn't tokenize text at all. Instead, it treats the entire input as a single token and does not break it into individual tokens. The `keyword` analyzer is often used for fields containing email addresses, URLs, or product IDs and in other cases where tokenization is not desirable. 
+
+## Example
+
+Use the following command to create an index named `my_keyword_index` with a `keyword` analyzer:
+
+```json
+PUT /my_keyword_index
+{
+  "mappings": {
+    "properties": {
+      "my_field": {
+        "type": "text",
+        "analyzer": "keyword"
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Configuring a custom analyzer
+
+Use the following command to configure an index with a custom analyzer that is equivalent to the `keyword` analyzer:
+
+```json
+PUT /my_custom_keyword_index
+{
+  "settings": {
+    "analysis": {
+      "analyzer": {
+        "my_keyword_analyzer": {
+          "tokenizer": "keyword"
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Generated tokens
+
+Use the following request to examine the tokens generated using the analyzer:
+
+```json
+POST /my_custom_keyword_index/_analyze
+{
+  "analyzer": "my_keyword_analyzer",
+  "text": "Just one token"
+}
+```
+{% include copy-curl.html %}
+
+The response contains the generated tokens:
+
+```json
+{
+  "tokens": [
+    {
+      "token": "Just one token",
+      "start_offset": 0,
+      "end_offset": 14,
+      "type": "word",
+      "position": 0
+    }
+  ]
+}
+```