doc: update installer logo assets to the new branding

[zeexzeex]

Summary

Updates the installer logo assets in doc/ to the standardized
Node.js branding (openjs-foundation/artwork#24), and adds
tools/generate-installer-assets.sh so the assets can be regenerated
reproducibly — as noted in the issue, the original generation process
was lost.

Fixes: #60213

What changed

File	Consumed by	Dimensions
doc/thin-white-stripe.jpg	Windows MSI banner (WixUIBannerBmp)	493×58
doc/full-white-stripe.jpg	Windows MSI first/last dialog (WixUIDialogBmp)	493×312
doc/osx_installer_logo.png	macOS installer background	180×361 (transparency preserved)
doc/osx_installer_logo_dark.png	macOS installer background (dark mode)	180×361 (new)
tools/generate-installer-assets.sh	—	new

The previous attempt (#60218) was closed because replacing the images
without matching the original dimensions would break the fixed-size
installer UI slots. To avoid that, the logo widths and offsets used by
the script were measured from the previous assets, so the layout is
pixel-identical — only the logo artwork and the brand green change.

Dark mode support

The macOS installer declares only a light <background>, so in dark
mode no background was shown at all (dark mode postdates these
assets). This PR adds a dark variant generated from the official
nodejs-logo-stacked-color-dark_background.svg and references it via
<background-darkAqua> in distribution.xml.tmpl.

How the assets are generated

tools/generate-installer-assets.sh:

• downloads the standardized stacked logo SVGs from the artwork
  repository, pinned to a commit SHA for reproducibility;
• renders them with rsvg-convert and composites with ImageMagick
  onto each canvas (white for the MSI bitmaps, transparent for macOS);
• verifies output dimensions and strips volatile metadata, so
  re-running the script produces byte-identical files.

The top stripe color was updated from the old brand green (#43853D)
to the current one (#5FA04E), matching the solid fill of the
standardized logo and https://nodejs.org/en/about/branding.

Verification

• Built the macOS installer locally on an ARM64 Mac
  (make DISTTYPE=custom CUSTOMTAG=test pkg) as suggested in chore: update logo asset #60218:
  the background renders correctly in both light and dark mode with
  no truncation or stretching — screenshots below.
• pkgutil --expand confirms the packaged background PNG is
  byte-identical to the new asset.
• The regenerated JPEGs keep the exact encoding properties the MSI
  already consumes (JPEG, same dimensions, sRGB, 8-bit; baseline
  instead of progressive, which is the more conservative variant for
  GDI+), so no format compatibility change is expected on Windows.
• I don't have a Windows build environment, so I'd appreciate a visual
  check of the MSI dialogs during review — the bitmaps keep the exact
  dimensions WixUI expects, so no layout change is anticipated.

Light	Dark