Add description to Custom events

[FrankHoli]

Feature request

Is your feature request related to a problem?

We are having problems to make our custom events understandable to all users of Posthog. The only information we are able to provide is through the naming convention and the properties. However, this proves to be insufficient for people with a less technical background.

Describe the solution you'd like

MUST: We would like to have the ability to give custom events a description to enrich the information.
COULD: We would like to add pictures to add designs and communicate which part of the app is exactly tracked

Describe alternatives you've considered

Creating own documentation. However, we fear that this will be difficult to maintain over time and there will be a div between Posthog and the documentation which will lead to confusion and less trust.

Additional context

Debug info

- [ ] PostHog Cloud, Debug information: [please copy/paste from https://us.posthog.com/settings/project-details#variables]
- [ ] PostHog Hobby self-hosted with `docker compose`, version/commit: [please provide]
- [ ] PostHog self-hosted with Kubernetes (deprecated, see [`Sunsetting Kubernetes support`](https://posthog.com/blog/sunsetting-helm-support-posthog)), version/commit: [please provide]