doc: fix files mis-named as PNG files

There are a few non-PNG (JPEG and WebP) files that are being
name as PNG files. This causes pdflatex/latexmk to fail due
to them not actually being PNG files. So rename those files
with correct extensions and update the RST files.

Also converted the WebP file in JPEG as PDFLatex cannot parse
WebP image.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>