docs: Create with tutorial

Create docs, based on Sphinx with Commonmark, seeding it with https://forums.puri.sm/t/translations-and-virtual-touch-keyboards-tracking-localization/7669/48?u=dcz

doc/build.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+# Builds the documentation and places in the selected directory,
+# or the working directory.
+
+set -e
+
+SCRIPT_PATH="$(realpath "$0")"
+DOCS_DIR="$(dirname "$SCRIPT_PATH")"
+
+TARGET_DIR="${1:-./}"
+
+SPHINX=sphinx-build
+
+if ! which sphinx-build ; then
+    SPHINX=sphinx-build-3
+fi
+$SPHINX -b html "${DOCS_DIR}" "${TARGET_DIR}"