Include nginx repo installation in migration guide

[patschi]

First - thank you so much for taking up this work! Especially since the recent nginx CVE being released, ondrej's shutdown of the nginx mirror did hurt quite a bit. Thank you!

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This aside, when migrating I ran into a stupid migration human-mistake. For some reasons it just clicked too late in having to install the nginx repos first.

Hence, I'd suggest adding clear point/instructions to Moving from Sury to nginx‑modules for also adding the official nginx repo from here.

[bashgeek]

Thanks, good point! We just added it.

[patschi]

Cool! I'd probably also just link to nginx's official page - just for completness.

Also, any particular reason why using http:// for nginx repo and not https://?