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.

images/logo.svg

@@ -0,0 +1,34 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg width="128" height="128" viewBox="0 0 128 128" fill="none" xmlns="http://www.w3.org/2000/svg" role="img" aria-label="proton-stream">
+  <defs>
+    <linearGradient id="ps-face" x1="12" y1="12" x2="116" y2="112" gradientUnits="userSpaceOnUse">
+      <stop offset="0" stop-color="#A48BFF"/>
+      <stop offset="0.55" stop-color="#7552FF"/>
+      <stop offset="1" stop-color="#5B36F2"/>
+    </linearGradient>
+    <linearGradient id="ps-glyph" x1="0" y1="36" x2="0" y2="88" gradientUnits="userSpaceOnUse">
+      <stop offset="0" stop-color="#FFFFFF"/>
+      <stop offset="1" stop-color="#ECE6FF"/>
+    </linearGradient>
+    <!-- The play mark, cut into the blocks a revision is fetched in: each one
+         decrypts on its own, which is the whole reason this app can seek. -->
+    <mask id="ps-cut" maskUnits="userSpaceOnUse" x="0" y="0" width="128" height="128">
+      <path d="M50 41.5v41a6 6 0 0 0 9.1 5.1l34.2-20.5a6 6 0 0 0 0-10.3L59.1 36.4A6 6 0 0 0 50 41.5z" fill="#FFFFFF"/>
+      <g fill="#000000">
+        <rect x="63" y="0" width="5.5" height="128"/>
+        <rect x="78.5" y="0" width="5.5" height="128"/>
+      </g>
+    </mask>
+  </defs>
+
+  <!-- base: a rounded tile with a darker lower edge for depth -->
+  <rect x="12" y="16" width="104" height="100" rx="26" fill="#4424C0"/>
+  <rect x="12" y="12" width="104" height="100" rx="26" fill="url(#ps-face)"/>
+  <rect x="12.5" y="12.5" width="103" height="99" rx="25.5" stroke="#FFFFFF" stroke-opacity="0.2"/>
+
+  <!-- glyph shadow, then the glyph itself -->
+  <g transform="translate(0 3)">
+    <rect width="128" height="128" fill="#2E1596" fill-opacity="0.3" mask="url(#ps-cut)"/>
+  </g>
+  <rect width="128" height="128" fill="url(#ps-glyph)" mask="url(#ps-cut)"/>
+</svg>