refactor: remove references to legacy setting

The `--alpha` flag and `alpha = true/false` config file options remain
functional but references to them are removed as they are now superseded
by the output formats with alpha channels (rgba, hexa, hsla).
New users/configurations should use the relevant output formats. The
legacy flag/options remain active for backward-compatibility with older
configs/scripts.

README.md

@@ -84,20 +84,22 @@ You can define your preferred defaults in `~/.config/color-tool/config.toml`. Th
 
 ```toml
 [defaults]
-output = "hex"  # default output format(s)
+# Set any option to true to enable it by default when using the terminal
+output = "hex"  # default output format(s): hex, rgb, hsl, rgba, hsla, hexa, all
 json   = false  # output in JSON format
 swatch = false  # show color swatch in terminal
-name   = false  # fetch color name
+name   = false  # fetch color name from thecolorapi.com
 copy   = false  # copy result to clipboard
-pick   = false  # auto-launch picker
+pick   = false  # auto-launch color picker when invoked with no arguments
 notify = false  # show desktop notification
 
 [desktop]
-output = "hex"
-json   = false
-name   = false
-copy   = true
-notify = true
+# Defaults for --desktop mode (launched from the app menu; copy is always enabled by default)
+output = "hex"  # format to copy
+json   = false  # copy JSON format instead of plain text
+name   = false  # fetch color name (requires network)
+copy   = true   # copy result to clipboard
+notify = true   # show desktop notification with the copied value
 ```
 
 ## 🤝 Credits