Add keep words token filter docs #8064 (#8124)

* adding keep words token filter docs #8064

Signed-off-by: Anton Rubin <anton.rubin@eliatra.com>

* fixing vale errors

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* Update keep-words.md

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* updating parameter table

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* Update keep-words.md

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* Apply suggestions from code review

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* Apply suggestions from code review

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---------

Signed-off-by: Anton Rubin <anton.rubin@eliatra.com>
Signed-off-by: AntonEliatra <anton.rubin@eliatra.com>
Signed-off-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>
Co-authored-by: kolchfa-aws <105444904+kolchfa-aws@users.noreply.github.com>
Co-authored-by: Nathan Bower <nbower@amazon.com>
(cherry picked from commit 1bb7f3e)
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Author: 3 people

_analyzers/token-filters/index.md

@@ -33,7 +33,7 @@ Token filter | Underlying Lucene token filter|  Description
 [`hunspell`]({{site.url}}{{site.baseurl}}/analyzers/token-filters/hunspell/) | [HunspellStemFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/hunspell/HunspellStemFilter.html) | Uses [Hunspell](https://en.wikipedia.org/wiki/Hunspell) rules to stem tokens. Because Hunspell allows a word to have multiple stems, this filter can emit multiple tokens for each consumed token. Requires the configuration of one or more language-specific Hunspell dictionaries.
 `hyphenation_decompounder` | [HyphenationCompoundWordTokenFilter](https://lucene.apache.org/core/9_8_0/analysis/common/org/apache/lucene/analysis/compound/HyphenationCompoundWordTokenFilter.html) | Uses XML-based hyphenation patterns to find potential subwords in compound words and checks the subwords against the specified word list. The token output contains only the subwords found in the word list.
 [`keep_types`]({{site.url}}{{site.baseurl}}/analyzers/token-filters/keep-types/) | [TypeTokenFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/core/TypeTokenFilter.html) | Keeps or removes tokens of a specific type.
-`keep_word` | [KeepWordFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/miscellaneous/KeepWordFilter.html) | Checks the tokens against the specified word list and keeps only those that are in the list.
+[`keep_words`]({{site.url}}{{site.baseurl}}/analyzers/token-filters/keep-words/)  | [KeepWordFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/miscellaneous/KeepWordFilter.html) | Checks the tokens against the specified word list and keeps only those that are in the list.
 `keyword_marker` | [KeywordMarkerFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/miscellaneous/KeywordMarkerFilter.html) | Marks specified tokens as keywords, preventing them from being stemmed.
 `keyword_repeat` | [KeywordRepeatFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/miscellaneous/KeywordRepeatFilter.html) | Emits each incoming token twice: once as a keyword and once as a non-keyword.
 `kstem` | [KStemFilter](https://lucene.apache.org/core/9_10_0/analysis/common/org/apache/lucene/analysis/en/KStemFilter.html) | Provides kstem-based stemming for the English language. Combines algorithmic stemming with a built-in dictionary.

_analyzers/token-filters/keep-words.md

@@ -0,0 +1,92 @@
+---
+layout: default
+title: Keep words
+parent: Token filters
+nav_order: 190
+---
+
+# Keep words token filter
+
+The `keep_words` token filter is designed to keep only certain words during the analysis process. This filter is useful if you have a large body of text but are only interested in certain keywords or terms.
+
+## Parameters
+
+The `keep_words` token filter can be configured with the following parameters.
+
+Parameter | Required/Optional | Data type | Description
+:--- | :--- | :--- | :--- 
+`keep_words` |  Required if `keep_words_path` is not configured | List of strings | The list of words to keep.
+`keep_words_path` | Required if `keep_words` is not configured | String | The path to the file containing the list of words to keep.
+`keep_words_case` | Optional | Boolean | Whether to lowercase all words during comparison. Default is `false`.
+ 
+
+## Example
+
+The following example request creates a new index named `my_index` and configures an analyzer with a `keep_words` filter:
+
+```json
+PUT my_index
+{
+  "settings": {
+    "analysis": {
+      "analyzer": {
+        "custom_keep_word": {
+          "tokenizer": "standard",
+          "filter": [ "keep_words_filter" ]
+        }
+      },
+      "filter": {
+        "keep_words_filter": {
+          "type": "keep",
+          "keep_words": ["example", "world", "opensearch"],
+          "keep_words_case": true
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Generated tokens
+
+Use the following request to examine the tokens generated using the analyzer:
+
+```json
+GET /my_index/_analyze
+{
+  "analyzer": "custom_keep_word",
+  "text": "Hello, world! This is an OpenSearch example."
+}
+```
+{% include copy-curl.html %}
+
+The response contains the generated tokens:
+
+```json
+{
+  "tokens": [
+    {
+      "token": "world",
+      "start_offset": 7,
+      "end_offset": 12,
+      "type": "<ALPHANUM>",
+      "position": 1
+    },
+    {
+      "token": "OpenSearch",
+      "start_offset": 25,
+      "end_offset": 35,
+      "type": "<ALPHANUM>",
+      "position": 5
+    },
+    {
+      "token": "example",
+      "start_offset": 36,
+      "end_offset": 43,
+      "type": "<ALPHANUM>",
+      "position": 6
+    }
+  ]
+}
+```