Custom Blob Documents API documentation

[petarvukmirovic]

Dear documentation team,

Here I added documentation for new API that we introduced recently. The API allows registering
custom document types with corresponding editors.

Please make sure that the docs are made public only after 11.4 version of StudioPro is published.

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ quinntracy
❌ Petar Vukmirovic

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