Change HTML syntax to Markdown for images (3.9)

[josh-wong]

This PR changes how images are inserted into documentation.

Previously, the docs used HTML syntax to add images. However, Jekyll, the static site generator that our docs site is based on, cannot render those images because we have permalink: pretty enabled in the site configuration.

Note

permalink: pretty makes URLs a bit easier to read. For more information, see Built-in formats | Permalinks and Permalinks | Jekyll Style Guide.

Markdown syntax supports adding images, so I've updated the docs for branch 3.9, which is under support, to follow that standard.

[brfrn169]

LGTM! Thank you!

[komamitsu]

👍

[Torch3333]

LGTM, thank you!