Add Using Webhooks to PEM documentation

[dwicinas]

Added a section about Using webhooks to the Notifications topic in PEM.

Addresses https://enterprisedb.atlassian.net/browse/PEM-4330.

[github-actions[bot]]

🚀 Deployed on https://deploy-preview-5258--edb-docs-staging.netlify.app

[sonotley]

Without having reviewed the content in detail, the one thing that stands out to me is that the current location of the content feels inappropriate. The page in question is about technical details of how to use webhooks in PEM, then it suddenly starts talking about Slack. At minimum we should clearly explain that this is an example and we've just picked Slack because it's easy and commonly used. Perhaps it would make more sense to present it on a separate page as mini-tutorial.

[dwicinas]

@sonotley I made a small fix to emphasize that the new content was an example. Let me know what you think.

[sonotley]

Thanks @dwicinas. Now I look at it, I think the existing page has some issues with headers/sections. The content under ### Defining webhook SSL configurations is initially about SSL but goes on to talk about headers and payload without any further headings. I think we need some new headings something like

### Defining webhook SSL configuration
(content as far as the images)
### Defining the webhook headers
(content as far as the stuff about payload)
### Defing the webhook payload
(contents as far as the example)
### Example...

Let me know what you think.

[dwicinas]

@sonotley I revised the header structure based on your comments (which I liked). Please take a look. Thanks.

[sonotley]

I've just pushed a commit that sorts out the instructions for Slack, so this LGTM. Can I get a final review from docs team please?

[sonotley]

@nidhibhammar changes committed thanks. Ready to publish.