docs: reorganize documentation

- Simplify README
- Add docs/features.md (formats, roadmap)
- Add docs/usage.md (shortcuts)
- Keep docs/development.md (build instructions)

docs/images/usage.md

@@ -0,0 +1,46 @@
+# Usage
+
+## Keyboard Shortcuts
+
+### Navigation
+
+| Key   | Action            | Status  |
+|:------|:------------------|:--------|
+| `←`   | Previous document | working |
+| `→`   | Next document     | working |
+
+### Transformations
+
+| Key         | Action                         | Status  |
+|:------------|:-------------------------------|:--------|
+| `H`         | Flip horizontal                | working |
+| `V`         | Flip vertical                  | working |
+| `R`         | Rotate clockwise (90°)         | working |
+| `Shift + R` | Rotate counter-clockwise (90°) | working |
+
+### Zoom
+
+| Key       | Action                     | Status  |
+|:----------|:---------------------------|:--------|
+| `+` / `=` | Zoom in                    | partial |
+| `-`       | Zoom out                   | partial |
+| `1`       | Reset zoom (100% / actual) | working |
+| `F`       | Fit image to window        | working |
+
+### Pan
+
+| Key        | Action             | Status              |
+|:-----------|:-------------------|:--------------------|
+| `Ctrl + ←` | Pan left           | not yet implemented |
+| `Ctrl + →` | Pan right          | not yet implemented |
+| `Ctrl + ↑` | Pan up             | not yet implemented |
+| `Ctrl + ↓` | Pan down           | not yet implemented |
+| `0`        | Reset pan (center) | not yet implemented |
+
+### Modes
+
+| Key | Action                 | Status              |
+|:----|:-----------------------|:--------------------|
+| `C` | Toggle crop mode       | not yet implemented |
+| `S` | Toggle scale mode      | not yet implemented |
+| `A` | Toggle anotation mode  | not yet implemented |