Add Aegir to the list of providers in the "More ways to try Backdrop" page (when support for Backdrop has been added)

[klonos]

https://backdropcms.org/try-backdrop lists several ways to give Backdrop a go via various methods/vendors. With the recent announcement from the Aegir folks to support Backdrop + replace their D7-based admin dashboard backend with Backdrop (also see backdrop/backdrop-issues#6453), I think that we should keep this task in our radar and get it done when it's time.

[klonos]

cc'ing @omega8cc 😉