Bullseye release notes / install instructions for alternative inits

[Matthew Vernon]

Hi,

I have now updated https://wiki.debian.org/Init with newer instructions; 
I've sent both #992025 and #992034 smaller patches that are largely 
links to this wiki page, in the hope we can get _something_ included :(

If you have improvements to the wiki page, do please go ahead and edit it :)

Regards,

Matthew