docs: local publish script for Codeberg Pages; drop inert CI workflow

Codeberg doesn't execute Forgejo Actions (no runners), so the docs.yml
workflow never ran. Replace it with scripts/publish-docs.sh: build the
mdBook and force-push docs/book/ to the `pages` branch that Codeberg
Pages serves. Installs mdbook on demand, derives the Pages URL from the
origin remote, and honors CODEBERG_TOKEN for non-interactive auth.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.forgejo/workflows/docs.yml

@@ -1,44 +0,0 @@
-# Build the mdBook docs and publish them to Codeberg Pages.
-#
-# Codeberg serves the `pages` branch of a repo (or a dedicated `pages`
-# repo) at https://<user>.codeberg.page/<repo>/. This workflow builds the
-# book on a push to main that touches docs/ and pushes the rendered HTML
-# to the `pages` branch.
-name: docs
-
-on:
-  push:
-    branches: [main]
-    paths:
-      - 'docs/**'
-      - '.forgejo/workflows/docs.yml'
-  workflow_dispatch:
-
-jobs:
-  build:
-    runs-on: docker
-    container:
-      image: rust:1.85-bookworm
-    steps:
-      - name: Checkout
-        uses: actions/checkout@v4
-
-      - name: Install mdBook
-        run: cargo install mdbook --no-default-features --features search
-
-      - name: Build the book
-        run: mdbook build docs
-
-      - name: Publish to the pages branch
-        # forgejo-release isn't right here; push the built docs/book/ to a
-        # `pages` branch that Codeberg Pages serves. Uses the repo token.
-        env:
-          TOKEN: ${{ secrets.PAGES_TOKEN }}
-        run: |
-          cd docs/book
-          git init -q
-          git config user.name "ci"
-          git config user.email "ci@localhost"
-          git add -A
-          git commit -q -m "docs build ${{ github.sha }}"
-          git push -f "https://x:${TOKEN}@codeberg.org/${{ github.repository }}.git" HEAD:pages