feat: update README and add documentation for installation, configuration, usage, and plugin development

docs/configuration.md

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+# Configuration
+
+Config file: `~/.config/k-launcher/config.toml`
+
+The file is optional — all fields have defaults and missing sections fall back to defaults automatically. Create it manually if you want to customize behavior.
+
+## Full Annotated Example
+
+```toml
+[window]
+width       = 600.0   # window width in logical pixels
+height      = 400.0   # window height in logical pixels
+decorations = false   # show window title bar / frame
+transparent = true    # allow background transparency
+resizable   = false   # allow manual resizing
+
+[appearance]
+# RGBA: r/g/b are 0–255 as floats, a is 0.0–1.0
+background_rgba  = [20.0, 20.0, 30.0, 0.9]    # main background
+border_rgba      = [229.0, 125.0, 33.0, 1.0]  # accent/border color
+border_width     = 1.0    # border thickness in pixels
+border_radius    = 8.0    # corner radius of the window
+search_font_size = 18.0   # font size of the search input
+title_size       = 15.0   # font size of result titles
+desc_size        = 12.0   # font size of result descriptions
+row_radius       = 4.0    # corner radius of result rows
+placeholder      = "Search..."  # search input placeholder text
+
+[search]
+max_results = 8   # maximum results shown at once
+
+[plugins]
+calc  = true   # math expression evaluator
+cmd   = true   # shell command runner (> prefix)
+files = true   # filesystem browser (/ or ~/ prefix)
+apps  = true   # XDG application launcher
+```
+
+## RGBA Format
+
+Colors use `[r, g, b, a]` arrays where:
+- `r`, `g`, `b` — red, green, blue channels as floats **0.0–255.0**
+- `a` — alpha (opacity) as a float **0.0–1.0**
+
+Example — semi-transparent white: `[255.0, 255.0, 255.0, 0.5]`