docs(replay): Add MIGRATION.md & add badges to README.md

[mydea]

The second part of #6383, this adds migration instructions for replay (coming from 0.6).

In addition, I also added the header & badges @Lms24 mentioned in the previous PR that I then totally forgot to add 😅

Not sure if I forgot anything relevant?

Rendered README.md
Rendered MIGRATION.md

[github-actions]

size-limit report 📦

Path	Size
@sentry/browser - ES5 CDN Bundle (gzipped + minified)	19.6 KB (-0.01% 🔽)
@sentry/browser - ES5 CDN Bundle (minified)	60.69 KB (0%)
@sentry/browser - ES6 CDN Bundle (gzipped + minified)	18.37 KB (+0.01% 🔺)
@sentry/browser - ES6 CDN Bundle (minified)	54.26 KB (0%)
@sentry/browser - Webpack (gzipped + minified)	20.13 KB (0%)
@sentry/browser - Webpack (minified)	65.83 KB (0%)
@sentry/react - Webpack (gzipped + minified)	20.16 KB (0%)
@sentry/nextjs Client - Webpack (gzipped + minified)	46.96 KB (0%)
@sentry/browser + @sentry/tracing - ES5 CDN Bundle (gzipped + minified)	26.44 KB (0%)
@sentry/browser + @sentry/tracing - ES6 CDN Bundle (gzipped + minified)	24.87 KB (+0.02% 🔺)
@sentry/replay ES6 CDN Bundle (gzipped + minified)	41.74 KB (-0.01% 🔽)
@sentry/replay - Webpack (gzipped + minified)	37.23 KB (0%)

[Lms24]

Great, thanks for doing this! Just had a few suggestions