Add convert-images2webp.ps1 script and documentation for image conversion to WebP format

docs/convert-images2webp.md

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+# convert-images.ps1
+
+This PowerShell script converts images in a directory or a single image file to WebP format in parallel.
+
+## Parameters
+
+| Parameter | Description |
+| --- | --- |
+| `-InputPath` | The path to the input directory containing images or a single image file. |
+| `-OutputDir` | The path to the directory where the converted WebP files will be saved. It will be created if it doesn't exist. |
+| `-Quality` | The quality setting for the WebP conversion (0-100). Lower values mean smaller files but potentially lower quality. Defaults to 50. |
+| `-Lossless` | Specifies whether to use lossless WebP compression ($true) or lossy compression ($false). Defaults to $true. |
+| `-MaxParallel` | The maximum number of image conversions to run concurrently. Defaults to 10. |
+
+## Description
+
+The script takes advantage of PowerShell's parallel processing capabilities to efficiently convert multiple image files. It supports various input formats including:
+- jpg/jpeg
+- png
+- gif
+- bmp
+- tiff
+
+The converted files are saved in WebP format, which often provides superior compression while maintaining quality.
+
+## Examples
+
+```powershell
+# Convert all images in a directory
+./convert-images.ps1 -InputPath .\covers -OutputDir .\covers-webp -Quality 50 -Lossless $true -MaxParallel 10
+
+# Convert a single image
+./convert-images.ps1 -InputPath .\my_image.png -OutputDir .\output -Quality 80 -Lossless $false
+```
+
+## Requirements
+
+- PowerShell 7.0 or higher (required for ForEach-Object -Parallel)
+- ImageMagick must be installed and accessible via the 'magick' command in your PATH