Merge pull request ClickHouse#965 from DanRoscigno/architecture

Architecture

docs/en/_snippets/_config-files.md

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+:::important best practices
+When configuring ClickHouse Server by adding or editing configuration files you should:
+- Add files to `/etc/clickhouse-server/config.d/` directory
+- Add files to `/etc/clickhouse-server/users.d/` directory
+- Leave the `/etc/clickhouse-server/config.xml` file as it is
+- Leave the `/etc/clickhouse-server/users.xml` file as it is 
+:::

docs/en/_snippets/_replication-sharding-terminology.md

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+## Terminology
+### Replica 
+A copy of data.  ClickHouse always has at least one copy of your data, and so the minimum number of **replicas** is one.  This is an important detail, you may not be used to counting the original copy of your data as a replica, but that is the term used in ClickHouse code and documentation.  Adding a second replica of your data provides fault tolerance. 
+
+### Shard
+A subset of data.  ClickHouse always has at least one shard for your data, so if you do not split the data across multiple servers, your data will be stored in one shard.  Sharding data across multiple servers can be used to divide the load if you exceed the capacity of a single server. The destination server is determined by the **sharding key**, and is defined when you create the distributed table. The sharding key can be random or as an output of a [hash function](https://clickhouse.com/docs/en/sql-reference/functions/hash-functions).  The deployment examples involving sharding will use `rand()` as the sharding key, and will provide further information on when and how to choose a different sharding key.
+
+### Distributed coordination
+ClickHouse Keeper provides the coordination system for data replication and distributed DDL queries execution. ClickHouse Keeper is compatible with Apache ZooKeeper.