Add mdbook documentation for the sync server

docs/.gitignore

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+book
+tmp

docs/README.md

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+This is an [mdbook](https://rust-lang.github.io/mdBook/index.html) book.
+Minor modifications can be made without installing the mdbook tool, as the content is simple Markdown.
+Changes are verified on pull requests.

docs/book.toml

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+[book]
+authors = ["Dustin J. Mitchell"]
+language = "en"
+multilingual = false
+src = "src"
+title = "TaskChampion Sync Server"
+
+[output.html]
+default-theme = "ayu"

docs/src/SUMMARY.md

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+# Summary
+
+- [Introduction](./introduction.md)
+- [Usage](./usage.md)
+- [Integration](./integration.md)

docs/src/integration.md

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+# Integration
+
+TBD (pending Postgres support)

docs/src/introduction.md

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+# Introduction
+
+Taskchampion Sync-Server is an implementation of the TaskChampion [sync
+protocol][sync-protocol] server. It supports synchronizing Taskwarrior tasks
+between multiple systems.
+
+[sync-protocol]: https://gothenburgbitfactory.org/taskchampion/sync.html
+
+The project provides both pre-built images for common use-cases (see
+[usage](./usage.md)) and Rust libraries that can be used to build more
+sophisticated applications ([integration](./integration.md)).

docs/src/usage.md

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+# Usage
+
+## Running the Server
+
+The server is a simple binary that serves HTTP requests on a TCP port. The
+server does not implement TLS; for public deployments, the recommendation is to
+use a reverse proxy such as Nginx, haproxy, or Apache httpd.
+
+### Using Docker-Compose
+
+Every release of the server generates a Docker image in
+`ghcr.io/gothenburgbitfactory/taskchampion-sync-server`. The tags include
+`latest` for the latest release, and both minor and patch versions, e.g., `0.5`
+and `0.5.1`.
+
+The
+[`docker-compose.yml`](https://raw.githubusercontent.com/GothenburgBitFactory/taskchampion-sync-server/refs/tags/v0.6.1/docker-compose.yml)
+file in this repository is sufficient to run taskchampion-sync-server,
+including setting up TLS certificates using Lets Encrypt, thanks to
+[Caddy](https://caddyserver.com/).
+
+You will need a server with ports 80 and 443 open to the Internet and with a
+fixed, publicly-resolvable hostname. These ports must be available both to your
+Taskwarrior clients and to the Lets Encrypt servers.
+
+On that server, download `docker-compose.yml` from the link above (it is pinned
+to the latest release) into the current directory. Then run
+
+```sh
+TASKCHAMPION_SYNC_SERVER_HOSTNAME=taskwarrior.example.com \
+TASKCHAMPION_SYNC_SERVER_CLIENT_ID=your-client-id \
+docker compose up
+```
+
+The `TASKCHAMPION_SYNC_SERVER_CLIENT_ID` limits the server to the given client
+ID; omit it to allow all client IDs.
+
+It can take a few minutes to obtain the certificate; the caddy container will
+log a message "certificate obtained successfully" when this is complete, or
+error messages if the process fails. Once this process is complete, configure
+your `.taskrc`'s to point to the server:
+
+```none
+sync.server.url=https://taskwarrior.example.com
+sync.server.client_id=your-client-id
+sync.encryption_secret=your-encryption-secret
+```
+
+The docker-compose images store data in a docker volume named
+`taskchampion-sync-server_data`. This volume contains all of the task data, as
+well as the TLS certificate information. It will persist over restarts, in a
+typical Docker installation. The docker containers will start automatically on
+system startup. See the docker-compose documentation for more information.
+
+### Running the Binary
+
+The server is configured with command-line options. See
+`taskchampion-sync-server --help` for full details.
+
+The `--listen` option specifies the interface and port the server listens on.
+It must contain an IP-Address or a DNS name and a port number. This option is
+mandatory, but can be repeated to specify multiple interfaces or ports. This
+value can be specified in environment variable `LISTEN`, as a comma-separated
+list of values.
+
+The `--data-dir` option specifies where the server should store its data. This
+value can be specified in the environment variable `DATA_DIR`.
+
+By default, the server will allow all clients and create them in the database
+on first contact. There are two ways to limit the clients the server will
+interact with:
+
+- To limit the accepted client IDs, specify them in the environment variable
+`CLIENT_ID`, as a comma-separated list of UUIDs. Client IDs can be specified
+with `--allow-client-id`, but this should not be used on shared systems, as
+command line arguments are visible to all users on the system. This convenient
+option is suitable for personal and small-scale deployments.
+
+- To disable the automatic creation of clients, use the `--no-create-clients`
+flag or the `CREATE_CLIENTS=false` environment variable. You are now
+responsible for creating clients in the database manually, so this option is
+more suitable for large scale deployments.
+
+The server only logs errors by default. To add additional logging output, set
+environment variable `RUST_LOG` to `info` to get a log message for every
+request, or to `debug` to get more verbose debugging output.
+