swh-docs: Add Elasticsearch documentation

docs/elasticsearch.rst

+==============
+Elasticsearch
+==============
+
+Software Heritage uses an Elasticsearch cluster for long-term log storage.
+
+Hardware implementation
+=======================
+
+- 3x Xeon E3v6 (Skylake) servers with 32GB of RAM and 3x 2TB of hard drives each
+- 2x gigabit switches
+
+List of nodes
+-------------
+
+* esnode1.internal.softwareheritage.org.
+* esnode2.internal.softwareheritage.org.
+* esnode3.internal.softwareheritage.org.
+
+Architecture diagram
+====================
+
+.. graphviz:: images/elasticsearch.dot
+
+Per-node storage
+================
+
+- one root hard drive with a small filesystem
+- 3x 2TB hard drives in RAID0
+- xfs filesystem on this volume, mounted on */srv/elasticsearch*
+
+Remark
+======
+
+The root hard drive of the Elasticsearch nodes is also used to
+store an ext4 `Kafka` dedicated filesystem mounted on */srv/kafka* .