{gif, img}2webp: add page (#14024)

* Create gif2webm.md

* Create img2webp.md

* Rename gif2webm.md to gif2webp.md

pages/linux/gif2webp.md

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+# gif2webp
+
+> Convert a GIF image to WebP.
+> More information: <https://developers.google.com/speed/webp/docs/gif2webp>.
+
+- Convert a GIF image to WebP:
+
+`gif2webp {{path/to/image.gif}} -o {{path/to/image.webp}}`

pages/linux/img2webp.md

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+# img2webp
+
+> Convert an image to WebP.
+> More information: <https://developers.google.com/speed/webp/docs/img2webp>.
+
+- Convert an image to WebP:
+
+`img2webp {{path/to/image}} -o {{path/to/image.webp}}`