Add 'generic' platform to documentation

[jtdub]

Problem

The code supports a generic platform as a fallback in every _get_platform() method and it is a valid hier_config.Platform enum value. However, the README "Supported Platforms" section and the platform documentation only list five platforms (Cisco IOS, NX-OS, IOS-XR, Juniper Junos, Arista EOS).

Users won't know they can use generic for unsupported platforms.

Suggested Fix

Add Generic (generic) to the supported platforms list in the README and docs, with a note that it provides basic functionality without platform-specific rules.