Refactor bastardkb/dilemma/3x5_2 (#25462)

* Refactor bastardkb/dilemma/3x5_2

- Rename splinky -> promicro as the DIY PCB supports a Pro Micro
  compatible development board
    - Update keyboard aliases accordingly
- Migrated shared configuration to top level 3x5_2 directory
- Migrate keymap to JSON
- Migrate miscellaneous configuration to JSON
- Remove configuration that trends towards user-specific (CRC Table)
- Use short SPDX license headers
- Tidy up readme and make note of differences between PCBs

In short, the only difference between these two revisions (assembled vs.
promicro) is SPI vs. I2C (respectively) is used for the Cirque Trackpad.

* Migrate promicro configuration to AVR

The code in dilemma.c that reinitialises the ADC capable pins on RP2040
as digital inputs is not needed. Per section RP2040-E6 (p.630) of RP2040
datasheet, it appears digital inputs are *not* disabled, and this should
be handled internally anyway in matrix initialisation

data/mappings/keyboard_aliases.hjson

@@ -68,6 +68,9 @@
     "bakeneko80": {
         "target": "kkatano/bakeneko80"
     },
+    "bastardkb/dilemma/3x5_2/splinky": {
+        "target": "bastardkb/dilemma/3x5_2/promicro"
+    },
     "bastardkb/scylla/v2/elitec": {
         "target": "bastardkb/scylla/promicro"
     },