Document migrations setup

[jbmoelker]

Should include:

• using environments
• creating / autogenerating migrations (could add a migrations:create script to help out)
• use maintenance mode (could use script npx datocms maintenance:on|off, see datocms docs)
• apply migrations (could use a migrations:apply script to help out)
• promoting environments (could use a migrations:promote script to help out, see promote)
• refer to this in docs/upgrading.md as performing a migration is required in case of new migration files.

[jbmoelker]

Verified this works npx jiti scripts/create-migration.ts where scripts/create-migration.ts:

import { exec } from 'node:child_process';
import { datocmsEnvironment } from '../datocms-environment';

exec(`npx datocms migrations:new --autogenerate=${datocmsEnvironment} ${datocmsEnvironment}`, (error, stdout, stderr) => {
  if (error) {
    console.error(`exec error: ${error}`);
    return;
  }
  console.log(`stdout: ${stdout}`);
});