Add community mirror

[inge4pres]

The mirror is geographically redundant and fast thanks to Cloudflare CDN.
Instructions to replicate the same setup are also added in MIRRORS.md.

[linusg]

The "How to use" and "Why choose zigmirror.com?" sections on the marketing website are not in spirit of the Zig community mirror list IMO. Statements like "The fastest community mirror to download Zig binaries" ought to be backed up with numbers.

[inge4pres]

Hey @linusg 👋🏼 thanks for the hint, the website is simply a display page that show the property of the mirror - it's not meant much for marketing but for displaying how it's done, and why.

I actually re-run the numbers since the PR was implemented and noticed the latency and bandwidth of other mirrors have a similar latency (they can download a cached tarball in ~2 second, and uncached in ~12 secs.)
As for availability, it falls back to the Cloudflare SLAs for web and R2, as there are no other components involved, it's a completely managed service.

Let me know if I should update the website, which is not in this repo, for the PR to be accepted.
The failing check in CI does not seem related to the mirror implementation, which I had tested with the check-mirror script.

Here's what I used now just now to check the latency (for cached tarballs)

☁  www.ziglang.org [main] ⚡  curl -iLO https://zigmirror.com/zig-0.14.1.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
  0     0    0     0    0     0      0      0 --:--:-- --:--:-- --:--:--     0
100 16.9M  100 16.9M    0     0  6589k      0  0:00:02  0:00:02 --:--:-- 9080k
☁  www.ziglang.org [main] ⚡  curl -iLO https://pkg.machengine.org/zig/zig-0.14.1.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 16.9M  100 16.9M    0     0  2309k      0  0:00:07  0:00:07 --:--:-- 2071k
☁  www.ziglang.org [main] ⚡  curl -iLO https://zigmirror.hryx.net/zig/zig-0.14.1.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 16.9M  100 16.9M    0     0  2284k      0  0:00:07  0:00:07 --:--:-- 3604k
☁  www.ziglang.org [main] ⚡  curl -iLO https://zig.florent.dev/zig-0.14.1.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 16.9M  100 16.9M    0     0  4004k      0  0:00:04  0:00:04 --:--:-- 4004k
☁  www.ziglang.org [main] ⚡  curl -iLO https://zig.mirror.mschae23.de/zig/zig-0.14.1.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 16.9M  100 16.9M    0     0  8931k      0  0:00:01  0:00:01 --:--:-- 8931k
☁  www.ziglang.org [main] ⚡  curl -iLO https://zigmirror.meox.dev/zig-0.14.1.tar.xz
  % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
                                 Dload  Upload   Total   Spent    Left  Speed
100 16.9M    0 16.9M    0     0  7551k      0 --:--:--  0:00:02 --:--:-- 7552k