Clarify wording around Slack webhook compatibility

[svelle]

We talk a lot about Slack webhook compatibility and interchangeability on this docs page: https://docs.mattermost.com/developer/webhooks-incoming.html

However with the release of the Blocks system we don't support slackwebhooks as much as we used to so we should definitely make that more clear on this page as it can lead to confusion and frustration when customers are wondering why slack webhooks are not being displayed correctly in Mattermost.

[cwarnermm]

Thanks, @svelle! This is a page that now formally lives on the developer docs site and is scheduled to be archived in the docs site. That being said, this is great feedback and updates for that page of content. Thanks so much for bringing it to our attention.

[cwarnermm]

@svelle - Would you be open to creating a PR to update the developer documentation to incorporate this information? (There's no need to update the product docs since that duplicate content is scheduled to be archived).