Are there Webhooks available for Sharepoint APIs which can be used to notify when there are changes in a document stored in Sharepoint.
Yes, if you are using SharePoint online, they are available for events corresponding to list item changes for a given SharePoint list or a document library.
They support the below async events:
ItemAdded
ItemUpdated
ItemDeleted
ItemCheckedOut
ItemCheckedIn
ItemUncheckedOut
ItemAttachmentAdded
ItemAttachmentDeleted
ItemFileMoved
ItemVersionDeleted
ItemFileConverted
References - Getting started with webhooks