Style inline images as blocks with a border

[chalin]

• Closes Treat images as figures by default, and add a border etc #2358 using styles only. Maybe we can consider use of a render hook later.
• Ensures that image elements in markdown are treated as block-level elements
• Adds a light border around images, which is necessary (in most cases) when the image background is white. The border around barker images isn't a problem in general.
• Supports an easy way to declare that an image:
  • Shouldn't have a border by adding {.border-0} after an image. There's an example of this in the PR.
  • Isn't a figure, by using the img-initial class.
• Closes Add a frame border to an image #2271

Preview:

• https://deploy-preview-2359--opentelemetry.netlify.app/blog/2022/otel-demo-app-nomad/#4--see-it-in-nomad - illustrates how most images benefit from a border. This was also the target of Add a frame border to an image #2271
• https://deploy-preview-2359--opentelemetry.netlify.app/blog/2022/why-and-how-ebay-pivoted-to-opentelemetry/ - shows inline images without borders
• https://deploy-preview-2359--opentelemetry.netlify.app/docs/instrumentation/cpp/ - for the inline image in the description
• https://deploy-preview-2359--opentelemetry.netlify.app/docs/concepts/data-collection/#components - more inline images

Screenshots

Before:

After:

[cartermp]

Holy notification batman. I approve!

[chalin]

LOL, thanks for the review and approval @cartermp! ✨