docs(breakpoints): add guide for adding custom breakpoints

[CaerusKaru]

Currently the only way to add custom breakpoints is to provide the BREAKPOINTS token manually. If a user wants to include the default breakpoints, they have to specifically redeclare them, which might not be obvious.

The proposed solution is to add a new CUSTOM_BREAKPOINT multi token that can be concatenated to the main breakpoints array (taken from the BREAKPOINTS token) on startup.

[CaerusKaru]

Looks like we already have CUSTOM_BREAKPOINTS_PROVIDER_FACTORY, so this issue is going to track adding a guide for custom breakpoints.

[CaerusKaru]

This has been added as part of #670, closing.

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