Initial rename of Minetest to Luanti

[rollerozxa]

Renames most occurences of Minetest on the landing page to Luanti.

Some occurences have been left alone such as mentions to press resources that haven't been renamed yet. I left them not renamed as a reminder.

A "(formerly Minetest)" is added to the title of the index page.

I also added a note on the download page to reduce any confusion before 5.10.0 is out and the rename comes to the engine, so it can be merged right away:

Bikeshedding Feedback on the phrasing welcome, but I assume it can also be removed once 5.10.0 comes out which would be soon.

[appgurueu]

Otherwise looks fine. I've looked for occurrences where a rename might have been missed, but the vast majority are links that can't be updated yet.

_data/press.yml:    para: A flyer to promote Minetest.
_data/press.yml:    para: A leaflet to promote Minetest in Education.
_data/press.yml:    para: A showcase of the content Minetest has to offer.

though it is probably fine to leave them as-is for now since they (well, except for the last one) refer to past engine versions and are more consistent with the material this way.

[sfan5]

I also added a note on the download page to reduce any confusion before 5.10.0 is out and the rename comes to the engine, so it can be merged right away:

Maybe we should just merge this right before the 5.10 release then?

[appgurueu]

LGTM

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Maybe we should just merge this right before the 5.10 release then?

Question is how problematic it is to have the website be outdated (and thus also somewhat unsuitable for luanti.org) in the meantime. I'd be in favor of merging this now.

[rollerozxa]

I was thinking this would be merged right away so the landing page could be moved to luanti.org, but if there will be a release in a couple days I guess it wouldn't matter.

[rollerozxa]

I was hoping this would be merged like a week ago so we could have everything ready for the release, but I guess we're doing everything at once instead.