Update markdown to use absolute image paths, fixing broken images

Update markdown to use absolute image paths, fixing broken images

Markdown can be served with or without a trailing slash but when a
trailing slash is present, relative image links were broken.

The following example works as expected:
url: https://skia.org/dev/design/pdftheory
image url: PdfLogicalDocumentStructure.png
resolves to: https://skia.org/dev/design/PdfLogicalDocumentStructure.png

The following example was broken before this patch:
url: https://skia.org/dev/design/pdftheory/
image url: PdfLogicalDocumentStructure.png
resolves to: https://skia.org/dev/design/pdftheory/PdfLogicalDocumentStructure.png

The PdfLogicalDocumentStructure.png file lives in /dev/design/, not
/dev/design/pdftheory/. This patch updates all images in our markdown
files to be served with the full path so a trailing slash no longer
breaks images.

I hit this bug when promoting the new Skia debugger:
https://groups.google.com/a/chromium.org/d/msg/paint-dev/QJ8xpu8dNyA/F4KX2O4ZBwAJ

BUG=skia:5493
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2128013003
NOTRY=true
DOCS_PREVIEW= https://skia.org/?cl=2128013003

Review-Url: https://codereview.chromium.org/2128013003