Fix paths after the docs/Documents merge

Every image and link pointed at docs/, which no longer exists - the README
rendered with six broken images. Repoints them at Documents/, updates the
board table to the real Schematics/Left|Right|Numpad tree, and renames
firmware/ to Firmware/ to match the capitalised convention.

Documents/README.md now describes the whole folder rather than just the
datasheets, since the diagrams and BOM live there too.

Co-Authored-By: Claude Opus 5 <[email protected]>

Firmware/README.md

@@ -0,0 +1,9 @@
+# Firmware
+
+Nothing here yet. See the Firmware notes in the top-level README for what's
+already known — the `SPLIT_USB_DETECT` trap, the Num Lock resolve-at-press
+design, and the encoder config.
+
+**Licence: GPL-2.0-or-later.** Anything in this directory derives from QMK and
+inherits its licence. Unlike the hardware, it carries no NonCommercial
+restriction — the GPL does not permit adding one.