[custom] Remove platforms

[kbx81]

Description:

SSIA

Note: Some guides/cookbooks will also need to be modified/rewritten (outside this PR).

Related issue (if applicable): fixes

Pull request in esphome with YAML changes (if applicable): esphome/esphome#8119

Checklist:

• 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.

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

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[jesserockz]

Can we add a redirect for every removed page eg:

/components/binary_sensor/custom* /guides/contributing#a-note-about-custom-components 301

[esphome]

Please take a look at the requested changes, and use the Ready for review button when you are done, thanks 👍

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