docs: replace thelia:install with the update script in the 3.0 update page

[anoziere]

The 3.0 update page told readers to apply database migrations with php Thelia thelia:install. That command is the initial installer: it calls Database::insertSql(), which replays setup/thelia.sql, and that file starts with DROP TABLE IF EXISTS on every table. Running it on a live shop destroys the data.

The page now points at local/setup/update.php, the script that actually exists for this (it reads thelia_version from the database and replays the update scripts in setup/update/sql/ from there), and carries a danger admonition about thelia:install.

Three other fixes on the same page:

• a project depends on thelia/thelia-skeleton, not thelia/thelia, so composer update thelia/thelia and composer require thelia/thelia:^3.1 update nothing;
• in production, cache:clear boots the container it is about to remove, so the page now documents rm -rf var/cache/prod followed by cache:warmup, and says why skipping the warmup returns a 500 on back-office pages that render a live component;
• module:refresh now says what it does, which is comparing module.xml against the version stored in the database and running the module's update().

Applied to both the current docs and the frozen 3.0 version. npm run build passes.