Add magnetometer calibration reference

[nliaudat]

Description:

Add magnetometer calibration reference with esphome code

**Related issue (if applicable): nothing

Pull request in esphome with YAML changes (if applicable): esphome/esphome# Checklist:

[ X] Branch: next is for changes and new documentation that will go public with the next ESPHome release. Fixes, changes and adjustments for the current release should be created against current.
[X ] Link added in /index.rst when creating new documents for new components or cookbook. - Not applicable

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[github-actions[bot]]

There hasn't been any activity on this pull request recently. This pull request has been automatically marked as stale because of that and will be closed if no further activity occurs within 7 days. Thank you for your contributions.