DEVDOCS-6471 - Add context to Webhooks specification (#1097)

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# [DEVDOCS-6471]


## What changed?
* Added more explanatory content to Webhooks doc opening

## Release notes draft
* Clarified webhooks usage. Directed users to guide docs for more
in-depth coverage.

## Anything else?

ping { @dev-docs-team }


[DEVDOCS-6471]:
https://bigcommercecloud.atlassian.net/browse/DEVDOCS-6471?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

Author: bc-terra