Add doc comments to all crates

[tcr]

Our generated crates on docs.rs need much more documentation, in the form of crate and module-level /// comments. We should flesh these out so users can consult the docs without having to read the source.

• [ ] tessel crate
• [ ] accel-mma84 crate
• [ ] ble crate
• [ ] climate-si7020 crate
• [ ] relay-mono crate
• [ ] servo-pca9685 crate