Documentation

Add a page showing the overview screens of the RCT Power app and which OIDs are used to display the values.
Mention that some behaviours and the entire content of the registry is an implementation detail of the vendor and are
not mandated by the protocol itself and as such may not apply to other implementations.
New page Faults for interpreting the fault[*].flt responses in RCT devices.
Split documentation by moving the vendor-specific (aka RCT Inverter) parts to their own section (Registry, Faults and
App mapping).
Protocol overview:
- Add section about lack of protocol-level error handling.
- Add note about EXTENSION commands, their structure is unknown.
- Document what is known about READ_PERIODICALLY and how it is supposed to work with vendor devices.
- Mention checksum algorithm.
- List all known commands as well as reserved ones.
- Describe plant communication, which has not been tested yet.
CLI invocation: Bash-completion activation changed with newer versions of Click.
CLI: Document why there is now write support in the CLI.

Features

Command has new functions is_write and is_response to help working with received frames.
Command learned about READ_PERIODICALLY, but it has not been tested yet.
Command learned about the PLANT_ equivalents of the other commands.

CLI: Implement support for Click 8.1 caused by API changes related to custom completions (Issue #17, PR #18).
Make setuptools happy again by adhering to PEP-508.

Click: Version 7.0 is the new minimum, it supported custom completion functions for the first time
Click: Version 8.1 is the new maximum, to guard against API changes during unconditionally updating the
dependencies.