Add docc catalog (#311)

Author: fabianfett

Sources/PostgresNIO/Docs.docc/index.md

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+# ``PostgresNIO``
+
+🐘 Non-blocking, event-driven Swift client for PostgreSQL built on [SwiftNIO].
+
+## Overview
+
+Features:
+
+- A ``PostgresConnection`` which allows you to connect to, authorize with, query, and retrieve results from a PostgreSQL server
+- An async/await interface that supports backpressure 
+- Automatic conversions between Swift primitive types and the Postgres wire format
+- Integrated with the Swift server ecosystem, including use of [SwiftLog].
+- Designed to run efficiently on all supported platforms (tested extensively on Linux and Darwin systems)
+- Support for `Network.framework` when available (e.g. on Apple platforms)
+ 
+## Topics
+
+### Connections
+
+- ``PostgresConnection``
+
+### Querying
+
+- ``PostgresQuery``
+- ``PostgresBindings``
+- ``PostgresRow``
+- ``PostgresRowSequence``
+- ``PostgresRandomAccessRow``
+- ``PostgresCell``
+- ``PreparedQuery``
+- ``PostgresQueryMetadata``
+
+### Encoding and Decoding
+
+- ``PostgresEncodable``
+- ``PostgresEncodingContext``
+- ``PostgresDecodable``
+- ``PostgresDecodingContext``
+- ``PostgresArrayEncodable``
+- ``PostgresArrayDecodable``
+- ``PostgresJSONEncoder``
+- ``PostgresJSONDecoder``
+- ``PostgresDataType``
+- ``PostgresNumeric``
+
+### Notifications
+
+- ``PostgresListenContext``
+
+### Errors
+
+- ``PostgresError``
+- ``PostgresDecodingError``
+
+### Deprecated
+
+These types are already deprecated or will be deprecated in the near future. All of them will be 
+removed from the public API with the next major release. 
+
+- ``PostgresDatabase``
+- ``PostgresData``
+- ``PostgresDataConvertible``
+- ``PostgresQueryResult``
+- ``PostgresJSONCodable``
+- ``PostgresJSONBCodable``
+- ``PostgresMessageEncoder``
+- ``PostgresMessageDecoder``
+- ``PostgresRequest``
+- ``PostgresMessage``
+- ``PostgresMessageType``
+- ``SASLAuthenticationManager``
+- ``SASLAuthenticationMechanism``
+
+[SwiftNIO]: https://github.com/apple/swift-nio
+[SwiftLog]: https://github.com/apple/swift-log