Add documentation for visit.meta object

[daun]

Description

Document new visit.meta object for passing along custom data:

swup.hooks.on('visit:start', (visit) => {
  visit.meta.lorem = 'ipsum';
});

swup.hooks.on('page:view', (visit) => {
  console.log(visit.meta.lorem); // 'ipsum'
});

Checks

• The PR is submitted to the master branch
• The code was linted before pushing (npm run lint)
• All tests are passing (npm run test)
• New or updated tests are included
• The documentation was updated as required

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