refactor: rename firmware from Plus to Solo

- FIRMWARE_VERSION: v1.15-plus.1 → v1.15-solo.1
- FIRMWARE_PLUS_BUILD → FIRMWARE_SOLO_BUILD in all platformio.ini and UITask.cpp
- docs/plus_features/ → docs/solo_features/ (all sub-docs moved)
- README: update all doc links plus new Documentation section
- GitHub Actions: extract GIT_TAG_VERSION in each build job (was missing,
  causing empty FIRMWARE_VERSION in builds); rename output files to
  solo-VERSION-oled.uf2 / solo-VERSION-eink.uf2; release title → "Solo VERSION"

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.github/workflows/build-wio-tracker-l1-firmwares.yml

@@ -24,11 +24,17 @@ jobs:
       - name: Setup Build Environment
         uses: ./.github/actions/setup-build-environment
 
+      - name: Extract Version from Git Tag
+        run: echo "GIT_TAG_VERSION=${GITHUB_REF_NAME}" >> $GITHUB_ENV
+
       - name: Build Dual OLED Firmware
         env:
           FIRMWARE_VERSION: ${{ env.GIT_TAG_VERSION }}
         run: /usr/bin/env bash build.sh build-firmware WioTrackerL1_companion_dual
 
+      - name: Rename OLED firmware
+        run: mv out/*.uf2 "out/solo-${{ env.GIT_TAG_VERSION }}-oled.uf2" 2>/dev/null || true
+
       - name: Upload OLED Firmware
         uses: actions/upload-artifact@v4
         with:
@@ -45,11 +51,17 @@ jobs:
       - name: Setup Build Environment
         uses: ./.github/actions/setup-build-environment
 
+      - name: Extract Version from Git Tag
+        run: echo "GIT_TAG_VERSION=${GITHUB_REF_NAME}" >> $GITHUB_ENV
+
       - name: Build Dual E-ink Firmware
         env:
           FIRMWARE_VERSION: ${{ env.GIT_TAG_VERSION }}
         run: /usr/bin/env bash build.sh build-firmware WioTrackerL1Eink_companion_dual
 
+      - name: Rename E-ink firmware
+        run: mv out/*.uf2 "out/solo-${{ env.GIT_TAG_VERSION }}-eink.uf2" 2>/dev/null || true
+
       - name: Upload E-ink Firmware
         uses: actions/upload-artifact@v4
         with:
@@ -80,7 +92,7 @@ jobs:
       - name: Create Release
         uses: softprops/action-gh-release@v2
         with:
-          name: Wio Tracker L1 Firmware ${{ env.GIT_TAG_VERSION }}
+          name: Solo ${{ env.GIT_TAG_VERSION }}
           body: ""
           draft: true
           files: out/*.uf2