Document process for customizing installer assets

_Originally posted by @jamesobutler in https://github.com/Slicer/Slicer/pull/8192#issuecomment-2622905076_

I styled the Slicer header and welcome image bmp by hand. Attached are the gimp project files ([GimpImageProject.zip](https://github.com/user-attachments/files/18594195/GimpImageProject.zip)) that I used to export the final bmp. For Slicer's welcome image I actually applied a bilinear gradient with a lighter region around the logo to make the text readable, with the Slicer grey color going towards the top and bottom of the image. For my Slicer custom app, the welcome image is a much more complex background from our company's various resources and doesn't actually utilize the application logo in that space.

Reference: https://nsis.sourceforge.io/Docs/Modern%20UI/Readme.html
- NSIS recommends the header image MUI_HEADERIMAGE_BITMAP to be 150x57 pixels in size
- NSIS recommends the welcome image MUI_WELCOMEFINISHPAGE_BITMAP and MUI_UNWELCOMEFINISHPAGE_BITMAP to be 164x314 pixels in size
- The bmp images were exported with the following options.

Start with the attached GimpImageProject.zip and the referenced NSIS Modern UI Readme. Locate the repository's appropriate documentation entry point, then document how to customize and export the installer header and welcome BMP assets, including the recommended dimensions and export options. Done means a newcomer can reproduce the assets for a custom Slicer application.

Written by the indexing model from the issue text.