docs: list ProMicro solo build, document CARDKB_I2C, update release notes

- README's env table and external_keyboard.md's device-support table both
  get a ProMicro row -- missed when the PR #31 merge landed since neither
  was in scope for that commit.
- external_keyboard.md and build_flags.md now cover CARDKB_I2C alongside
  ENV_PIN_SDA/ENV_PIN_SCL: the PR generalised CardKB bus selection to a
  single macro after build_flags.md was first written, so the doc only had
  the older (still valid, but no longer the only) form.
- release-notes.md: added the ProMicro solo build and the new Build Flags
  reference page to v1.26's What's New.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/solo_features/external_keyboard.md

@@ -23,6 +23,7 @@ see [Built-in keyboards](#built-in-keyboards-cardputer-adv-t-echo-lite--keyshiel
 | Heltec V4 *(experimental)* | ✅ solder to free GPIOs | ✅ solder to free GPIOs |
 | M5Stack Cardputer ADV *(experimental)* | — | built-in keyboard instead, see below |
 | LilyGO T-Echo Lite + KeyShield *(experimental)* | — | built-in keypad instead, see below |
+| ProMicro (nRF52840) *(experimental)* | ✅ shares the primary I2C bus (D8/D7) — no free pins for a second bus | — |
 
 ---
 
@@ -35,6 +36,11 @@ for it once at boot — nothing to enable in Settings.
 The same bus is scanned for environment sensors, so a CardKB and a sensor can
 share it.
 
+On a board with no free pins for a second bus (ProMicro), CardKB instead
+shares the primary bus already used by the display/RTC — set `CARDKB_I2C=Wire`
+as a build flag rather than `ENV_PIN_SDA`/`ENV_PIN_SCL`. See
+[Build Flags](./build_flags.md) for both forms.
+
 ### Typing
 
 Printable characters insert straight at the cursor, bypassing the on-screen grid