Override keybinding using environment variable

[chrisdicarlo]

This pull request modifies the keybinding configuration, allowing for customization via an environment variable. The changes involve both documentation updates and modifications to the configuration file. It retains the existing default keybinding in the absence of the .env key.

The rationale for the change is to allow different developers in the same repository to customize the keybinding based on their personal preference and desktop environment without constantly changing the config file.

Example of change:

WIRE_SPY_KEYBINDING="ctrl.shift.y"