Improve webhooks docs

[m-murasovs]

It appears the docs are out of date and could do with improvement. A couple of people on Discord were confused, as reported by Paulo. Conversation: https://apifier.slack.com/archives/CQ96RHG2U/p1642786975000300

Also check: https://apifier.slack.com/archives/DUC52PG0J/p1647352424354999

Check out Discord to see what they need and check with Platform team how we can improve.

Probably move all the content to one page?

Also see: https://apifier.slack.com/archives/CQ96RHG2U/p1669482412125069

We need to document retries and exponential backoff, this is not described anywhere and users are surprised by this (Slack).

[fnesveda]

I was reading through the docs now, and I think the issues that people complained about are fixed:

• actions are listed
• variable interpolation is explained
• retries and exponential backoff are there
• the structure is better after Mára's restructuring

I will just close this, if it's needed we can reopen it.