docs: user guide pages, published to the GitHub wiki

docs/ gains pages for people using the app rather than building it:
getting started, installation, the user guide, account and sync,
Android and iOS, the command line, privacy, an FAQ and troubleshooting.
docs/README.md is the wiki's home page.

scripts/publish-wiki.sh, ported from proton-drive-linux, checks every
link in docs/ and README.md, rewrites links between pages to wiki page
names and copies the images they show. The Wiki workflow runs the check
on pull requests and publishes on every push to main.

images/ holds library, title and episode screenshots from the desktop
and Android apps, and the logo.

.github/workflows/wiki.yml

@@ -0,0 +1,68 @@
+name: Wiki
+
+# docs/ is the source of the GitHub wiki. Every change to the documentation
+# checks its links; a push to main publishes the result, so the wiki follows
+# main. Pull requests only check: nothing unmerged reaches the wiki. Run it from
+# the Actions tab to publish another ref, such as a release tag.
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'docs/**'
+      - 'images/**'
+      - 'README.md'
+      - 'scripts/publish-wiki.sh'
+      - '.github/workflows/wiki.yml'
+  pull_request:
+    paths:
+      - 'docs/**'
+      - 'images/**'
+      - 'README.md'
+      - 'scripts/publish-wiki.sh'
+      - '.github/workflows/wiki.yml'
+  workflow_dispatch:
+    inputs:
+      ref:
+        description: 'Tag or branch to publish'
+        required: false
+        default: 'main'
+        type: string
+
+permissions:
+  contents: read
+
+jobs:
+  check:
+    name: Check documentation links
+    runs-on: ubuntu-24.04
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          ref: ${{ inputs.ref || github.sha }}
+
+      - name: Check links
+        run: scripts/publish-wiki.sh --check "$RUNNER_TEMP"
+
+  publish:
+    name: Publish to the wiki
+    needs: check
+    # Pull requests never publish.
+    if: github.event_name != 'pull_request'
+    runs-on: ubuntu-24.04
+    concurrency:
+      group: wiki-publish
+      cancel-in-progress: false
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v7
+        with:
+          ref: ${{ inputs.ref || github.sha }}
+
+      - name: Publish
+        env:
+          WIKI_REMOTE: https://x-access-token:${{ github.token }}@github.com/${{ github.repository }}.wiki.git
+        run: |
+          git config --global user.name 'github-actions[bot]'
+          git config --global user.email '41898282+github-actions[bot]@users.noreply.github.com'
+          scripts/publish-wiki.sh --push "$RUNNER_TEMP"