design: a centred, captioned, attributed content image between paragraphs

There is no pattern on the site for a content image that sits between paragraphs of body text:
centred, at its natural aspect ratio, with a visible caption and attribution underneath.

The only image idioms the 2017 template gives us are:

• spotlight, which puts the image in a 35%-wide column with object-fit: cover, so the photo is
  centre-cropped at desktop;
• gallery, which is a thumbnail grid with a lightbox;
• the banner.

That came up on #174. The 2011 RWNYC team photo is a spotlight, so at desktop width the crop cuts
off the ringer on the far left, most of the one on the far right, and Anthony Bianco's watermark
along the bottom-left. At 375px the section stacks and the whole band is visible, so it is a
desktop-only crop. The alternative, a plain wrapper style1 with the photo inline and uncropped,
has no styling behind it today, which is why #174 shipped the spotlight and left this to be picked
up separately.

The RWNYC photo would be the first thing to move onto the new pattern once it lands.

Acceptance criteria:

• A Sass component for a centred content image with a caption and a credit line, in
  assets/sass/components/, following the existing component conventions.
• It preserves the image's aspect ratio at every breakpoint, with no object-fit: cover crop,
  and carries width/height so render.yml's ratio check passes.
• The caption and credit are visible text, not title= or hover-only, and the credit can carry
  a link to the source.
• It reads sensibly at 375px as well as desktop.
• Documented wherever a page author would look for it.

Start by reviewing the existing component conventions in assets/sass/components/ and the image ratio check in render.yml. Implement the centred content-image pattern with responsive, uncropped dimensions, visible caption and linked credit, then verify it at 375px and desktop widths and document it where page authors look for image patterns.