Modem component documentation

[oarcher]

Description:

Modem documentation.

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#6721

Checklist:

• [X] I am merging into next because this is new documentation that has a matching pull-request in esphome as linked above.
  or

• [ ] I am merging into current because this is a fix, change and/or adjustment in the current documentation and is not for a new component or feature.

• [x] Link added in /index.rst when creating new documents for new components or cookbook.

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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.