[generator] Add module docs for new API namespaces

Author: swallez

api_generator/docs/namespaces/autoscaling.md

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+Autoscaling APIs
+
+The [autoscaling feature](https://www.elastic.co/guide/en/elasticsearch/reference/master/xpack-autoscaling.html)
+enables an operator to configure tiers of nodes that self-monitor whether or not they need to scale based on an
+operator-defined policy.

api_generator/docs/namespaces/eql.md

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+EQL APIs
+
+[Event Query Language (EQL)](https://www.elastic.co/guide/en/elasticsearch/reference/master/eql.html) is a query
+language for event-based time series data, such as logs, metrics, and traces.
+
+

api_generator/docs/namespaces/logstash.md

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+Logstash APIs
+
+The [Logstash APIs](https://www.elastic.co/guide/en/elasticsearch/reference/master/logstash-apis.html) are used to
+manage pipelines used by Logstash Central Management.

api_generator/docs/namespaces/monitoring.md

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+Monitoring APIs
+
+The Elastic Stack [monitoring features](https://www.elastic.co/guide/en/elasticsearch/reference/master/monitor-elasticsearch-cluster.html)
+provide a way to keep a pulse on thehealth and performance of your Elasticsearch cluster.

api_generator/docs/namespaces/rollup.md

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+Rollup APIs
+
+The Elastic Stack [data rollup features](https://www.elastic.co/guide/en/elasticsearch/reference/master/xpack-rollup.html)
+provide a means to summarize and store historical data so that it can still be used for analysis, but at a fraction of
+the storage cost of raw data.

api_generator/docs/namespaces/searchable_snapshots.md

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+Searchable snapshot APIs
+
+[Searchable snapshots](https://www.elastic.co/guide/en/elasticsearch/reference/master/searchable-snapshots.html) let
+you reduce your operating costs by using snapshots for resiliency rather than maintaining replica shards within a
+cluster. This can result in significant cost savings for less frequently searched data.
+

api_generator/docs/namespaces/text_structure.md

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+Text structure APIs
+
+Determines the structure of text and other information that will be useful to import its contents to an Elasticsearch
+index.