README: change to official NixOS wiki

[Mic92]

Context: https://discourse.nixos.org/t/wiki-nixos-org-is-now-live/42570

[Omikhleia]

Greetings,

• [x] SILE uses "conventional commits", so it seems to me that you should amend[^1] your commit so that it starts with chore(readme): Change to official NixOS wiki

• [x] Moreover the old wiki (https://nixos.wiki/wiki/Flakes#Installing_flakes, if I understand correctly), is also mentioned in the SILE manual (file documentation/c02-gettingstarted.sil). This would need to be fixed too, ideally is one more commit docs(manual): ...

• [x] Finally, there are a few .nix files in the repository mentioning https://nixos.wiki/wiki/Flakes#Using_flakes_project_from_a_legacy_Nix in comments. Shouldn't they be changed to use https://wiki.nixos.org/wiki/Flakes too? (But I can't find the referred section)

[^1]: E.g. git commit --amend, change the commit line and save, then git push --force

[Omikhleia]

N.B. I'm not a maintainer - I've approved the PR in order for CI the tests to be run and reported.