feat: add heroImage support to blog posts

[chadoh]

Currently has two bugs:

1. For the image to work with meta tags, it needs to live in public.
   And/or, for it to work with Markdown, it needs to live in public. I
   tried using relative paths to no avail. I didn't see much info in the
   docs about including relative images from Markdown.

   Since it lives in public, it's treated as a remote image by the
   @astrojs/image plugin [1]. This means we need to include an
   aspectRatio, which means we would need to make all blog posts have
   heroImages with equal ratios. The first image I threw in here uses a
   nonsensical ratio, because I did not understand this limitation when
   I made it.

2. The Picture component always adds a height attribute to the
   rendered element. The content has a max-width of about 800px, but I
   wanted to use a 1200px-wide image so it would look nicer on
   high-density displays. I'm not sure this is even optimized, but even
   if not, the resulting bug will still show up:

   The image ends up looking all squashed! The max-width of the image is
   the width of the content, while the height attribute sets something
   corresponding to the (hypothetical) width attribute. Since actual
   width is narrower than this 1200px, the height ends up mismatching.

   If there were a way to remove this attribute from the rendered
   element, it would look great!

[chadoh]

Looks like the second issue, which will make this usable, has been fixed by withastro/astro#4797. We can get this PR mergeable once it's released.