Add developer-facing documentation

[jwodder]

• Write a document covering the basics of the WebDAV protocol (Introduction to the WebDAV Protocol #151)
• Write a document outlining the program's architecture (Document program architecture #166)
• Write a document on the release process for this project (Document release process #149)
• Write a document describing the requirements for the Zarr manifests and their index (Document src/zarrman/ and its API requirements #163)

Place all documents in doc/.