Document moving an existing site into GOBLOG_DATA_DIR #670

Problem

#661 added GOBLOG_DATA_DIR for new installs. A site set up before it, with per-path bind mounts or files inside the container, has no documented way to adopt it.

What is needed

A short section in the README or UPGRADING.md: stop goblog, copy .env, the SQLite file, www/uploads, plugins/wasm, plugins/dynamic and themes/installed into the layout under the data directory (table in #661), set the variable, start. Note that an absolute sqlite_db path in .env is used as given, and that /wp-content/uploads still reads only from www.

A goblog migrate-data-dir command that does the copy would be better still.

Follow-up to #653 / #661.