Code to generate releases json static pages

[elcritch]

This PR adds static releases.json page. Note that install.md/install.html are parsed by tools so this PR doesn't modify it. However it'd be possible to update it.

It adds a formatted html releases.html, but it's not tested and may not be desirable as there's already install.html but I added it for completeness.

Folks want to download Nim release builds for ARM or macOS. I've raised it before, and others as well. Tools like grabnim or choosenim currently parse install.html or try to search github api for nightlies repo.

A releases.json file provides an easier and "official" way to find releases from nightlies for tools without needing to do the looksup everytime.

The json includes the sha256 digest, the github download url, updated time, and the nimlang download url if it exists.

The initial json output looks like:

{
  "2.2.6": {
    "linux_armv7l": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-10-31-version-2-2-ab00c56904e3126ad826bb520d243513a139436a/nim-2.2.6-linux_armv7l.tar.xz",
      "digest": "sha256:f634a67559ee54485d4007e3547555c94e661bfd9463e932c821129e0ba08577",
      "updated": "2025-10-31T04:46:50Z"
    },
    "linux_x32": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-10-31-version-2-2-ab00c56904e3126ad826bb520d243513a139436a/nim-2.2.6-linux_x32.tar.xz",
      "digest": "sha256:337e5474e6f198e6f66a5c7cf5d3e65cb533f672a65ad59826cb99a443c9e3bb",
      "updated": "2025-10-31T02:25:15Z",
      "nimlang_url": "https://nim-lang.org/download/nim-2.2.6-linux_x32.tar.xz"
    },
    "linux_x64": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-10-31-version-2-2-ab00c56904e3126ad826bb520d243513a139436a/nim-2.2.6-linux_x64.tar.xz",
      "digest": "sha256:38b8407f87d78bd207390051e4c76f38a45d0a26983cb262017c899b56ad8d06",
      "updated": "2025-10-31T02:23:51Z",
      "nimlang_url": "https://nim-lang.org/download/nim-2.2.6-linux_x64.tar.xz"
    },
    "macosx_arm64": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-10-31-version-2-2-ab00c56904e3126ad826bb520d243513a139436a/nim-2.2.6-macosx_arm64.tar.xz",
      "digest": "sha256:a62ff3c34a2eb3fc69866fcae05b32dc5c2b6668ecfb1d10d91ad72c0183fae5",
      "updated": "2025-10-31T02:19:40Z"
    },
    "macosx_x64": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-10-31-version-2-2-ab00c56904e3126ad826bb520d243513a139436a/nim-2.2.6-macosx_x64.tar.xz",
      "digest": "sha256:681baa5a857f369bfc1fe27e4f03c4931331aef1d587d2cfb2a7a97e2f5a35c3",
      "updated": "2025-10-31T02:30:55Z"
    },
    "windows_x32": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-10-31-version-2-2-ab00c56904e3126ad826bb520d243513a139436a/nim-2.2.6-windows_x32.zip",
      "digest": "sha256:00f02af5a62093480ed40a1092fb0f038c10fca907c68c35dc8212af9ba07369",
      "updated": "2025-10-31T02:44:13Z",
      "nimlang_url": "https://nim-lang.org/download/nim-2.2.6_x32.zip"
    },
    "windows_x64": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-10-31-version-2-2-ab00c56904e3126ad826bb520d243513a139436a/nim-2.2.6-windows_x64.zip",
      "digest": "sha256:557eed9a9193a3bc812245a997d678fd6dc2c2dec6cfa9ba664a16b310115584",
      "updated": "2025-10-31T02:42:00Z",
      "nimlang_url": "https://nim-lang.org/download/nim-2.2.6_x64.zip"
    },
    "source_tar": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-10-31-version-2-2-ab00c56904e3126ad826bb520d243513a139436a/nim-2.2.6.tar.xz",
      "digest": "sha256:657b0e3d5def788148d2a87fa6123fa755b2d92cad31ef60fd261e451785528b",
      "updated": "2025-10-31T02:08:51Z"
    }
  },
  "2.2.4": {
    "linux_arm64": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-04-22-version-2-2-f7145dd26efeeeb6eeae6fff649db244d81b212d/nim-2.2.4-linux_arm64.tar.xz",
      "updated": "2025-04-22T02:28:38Z"
    },
    "linux_armv7l": {
      "github_url": "https://github.com/nim-lang/nightlies/releases/download/2025-04-22-version-2-2-f7145dd26efeeeb6eeae6fff649db244d81b212d/nim-2.2.4-linux_armv7l.tar.xz",
      "updated": "2025-04-22T04:48:30Z"
    },
    ...
  },
  ...
}