Add xet protocol subsection pages, finish toctree

Author: rajatarya

docs/hub/xet/protocol/architecture.md

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+## Overall Xet Architecture
+
+(TODO: Add diagram here)
+
+These pages will describe the overall Xet architecture, including how to perform chunking & hashing of files for upload, how to process file reconstruction information when downloading, the specfics of the Xorb and Shard formats, and how deduplication is achieved by a client library.
+
+## Contents
+
+- [Content-Defined Chunking](./xet/protocol/chunking)
+- [Hashing Methods](./xet/protocol/hashing)
+- [File Reconstruction](./xet/protocol/file-reconstruction)
+- [Xorb Format](./xet/protocol/xorb)
+- [Shard Format](./xet/protocol/shard)
+- [Deduplication](./xet/protocol/deduplication)
+

docs/hub/xet/protocol/building-client.md

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+## Building a client library for Xet storage
+
+These pages explain how you can build a client library for Xet storage. Included in these pages are the steps involved in uploading & downloading files, the web service APIs involved, and how authentication and authorization works.
+
+## Contents
+
+- [Upload Protocol](./xet/protocol/upload-protocol)
+- [Download Protocol](./xet/protocol/download-protocol)
+- [CAS API](./xet/protocol/api)
+- [Authentication and Authorization](./xet/protocol/auth)
+- [Hub file conversion to Xet File ID](./xet/protocol/file-id)
+