fix: add example for preset_name and custom_participant_id in docs v2

[NeelDigonto]

Description

Add example for preset_name in add_participant docs v2

Path: /api#/operations/add_participant

Resolved issues

Closes API-257

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[vaibhavshn]

Hey, the api spec file is auto generated, hence this will eventually get overwritten.
We will try adding this from our end. Thank you for the PR!