docs: Add docs about internal event sending flow

[mydea]

This adds a docs page which outlines the flow that events go through when being processed.
I wrote this while trying to reason about the replay event sending - it can be quite hard to have an overview what is added/updated on an event where. This document hopefully helps a bit with that, for future reference.

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.71 KB (+0.03% 🔺)
@sentry/browser - ES5 CDN Bundle (minified)	61.11 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.5 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.64 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.28 KB (0%)
@sentry/browser - Webpack (minified)	66.33 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.3 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	47.3 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.7 KB (+0.01% 🔺)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	25.15 KB (+0.02% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	41.73 KB (0%)
@sentry/replay - Webpack (gzipped + minified)	38 KB (0%)

[Lms24]

Nice, thanks for writing this up! I have to look this up from time to time, so I think having such a flow comes in handy.