Replace reviung39 with reviung41_39

[jablko]

The REVIUNG39 outboard is slightly behind the REVIUNG41 (it's missing nice_nano_v2.overlay and MIRYOKU_LAYERS_FLIP), this change syncs them up.

The REVIUNG39 and 41 are almost the same --- you could flash both with the same firmware: reviewing the three ZMK REVIUNG39 PRs (zmkfirmware/zmk#622, zmkfirmware/zmk#695, and zmkfirmware/zmk#795), plus the REVIUNG39 vs. 41 QMK keyboards and the REVIUNG39 vs. 41 KiCad files, the only difference is that keymap positions RC(6,0), RC(6,1), and RC(6,2) vs. RC(6,1), RC(6,2), and RC(6,3) are mapped to SPC, ENT, and BSPC. Otherwise they're identical --- they share a common default keymap except for the outer thumbs.

So I added a reviung41_39 variant instead of duplicating boards/shields/reviung41 and used a matrix transform to account for the only difference. I repeated the RC(6,0) entry: map = <... RC(6,0) RC(6,0) RC(6,1) RC(6,2)>;, which expands to static uint32_t transform[] = {... [36] = 36, [36] = 37, [37] = 38, [38] = 39}; and transforms both boards' thumbs to the same keymap positions.

[manna-harbour]

Outboards are now documented at https://github.com/manna-harbour/miryoku_zmk#outboards. The zmk option has been replaced by https://github.com/manna-harbour/miryoku_zmk#outboard_branches. However, that should be used only where a keyboard requires additional code in a ZMK fork to operate. If the keyboard definition only is contained in a ZMK fork, https://github.com/manna-harbour/miryoku_zmk#repository-and-symlink-variables or https://github.com/manna-harbour/miryoku_zmk#outboard_modules should be used instead.