Fold README into develop.sentry.dev/self-hosted/

[chadwhitacre]

Better to have a single source of truth.

https://github.com/getsentry/onpremise/blob/master/README.md →
https://develop.sentry.dev/self-hosted/

[github-actions]

This issue has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Accepted, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀