Add more documentation

The current documentation is great as a reference for configuration parameters, but it would be better if we had more comprehensive documentation.

Since the project is in Python, it would make sense to use sphinx.

This issue is mostly here to aggregate various discussions that will arise when starting the documentation project.

Here is a first draft of a proposal for I'd like to see in the documentation:

- About PGHoard:
- What is it
- Main features / unique features
- Comparison with other tools
- Caveats
- Architecture
- Describe the client / server architecture
- List several common deployments pattern (same host ? centralized backup server ?)
- Build / Install
- Usage (including examples, basic and special)
- Configuration
- Backup
- Archiving
- Restoration
- Expiration policies (examples)
- How to monitor (alert files / replication slot / prometheus)
- Contributing
- Run from source
- Tests
- Internals : Storage / Compression / Encryption API specifications

I'll start working on a base structure and iterate from there.