Ensure no images are hotlinked

[XAMPPRocky]

@dtolnay brought up in #578 that we should probably go through and find any other hot linked images in the blog. This is a tracking issue for that, and to add documentation informing people that ideally they should place images and resources in static.

• Ensured blog has no hot links
• Added documentation (Document recommended way to include images #1570)