chore: stage I1 -- repo cleanup, minimal README

Working plans (docs/development, docs/design redesign notes) removed; the
two developer guides moved to docs/developer/. README cut to devices,
flashing, links and building (Wio Tracker L2 added); its feature list and
doc tables moved to docs/solo_features/README.md. RELEASE.md describes the
Solo release flow. Upstream's GitHub Pages workflow (docs.meshcore.io)
dropped, CLAUDE.md untracked, Buy Me a Coffee back in FUNDING.yml.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.github/FUNDING.yml

@@ -1 +1,2 @@
 github: meshcore-dev
+buy_me_a_coffee: MarekZegarek

.github/workflows/github-pages.yml

@@ -1,39 +0,0 @@
-name: Build and deploy Docs site to GitHub Pages
-
-on:
-  workflow_dispatch:
-  push:
-    branches:
-      - main
-
-permissions:
-  contents: write
-
-env:
-  FORCE_JAVASCRIPT_ACTIONS_TO_NODE24: true
-
-jobs:
-  github-pages:
-    runs-on: ubuntu-latest
-    steps:
-
-      - name: Checkout Repo
-        uses: actions/checkout@v6
-
-      - name: Setup Python
-        uses: actions/setup-python@v6
-        with:
-          python-version: '3.13'
-
-      - name: Build
-        run: |
-          pip install mkdocs-material
-          mkdocs build
-
-      - name: Deploy to GitHub Pages
-        uses: peaceiris/actions-gh-pages@v4.1.0
-        with:
-          github_token: ${{ secrets.GITHUB_TOKEN }}
-          cname: docs.meshcore.io
-          publish_dir: ./site
-          publish_branch: 'gh-pages'