Docs: Add "how to add images" to the Astro starter

https://github.com/Thinkmill/keystatic/blob/main/docs/src/content/pages/installation-astro.mdoc

^ It'd be great to include "how to add images and ensure they're resolved" to the astro docs, since there seems to be some awkwardness with how the paths are generated:

with src/content/posts/first-post.mdoc in the starter, uploading an image from the ui adds src/content/posts/first-post/content/image.jpeg and links ![](image.jpeg) in src/content/posts/first-post.mdoc, which Astro ends up not resolving.

Maybe worth changing the config / reccomending changing the config for a blog w/ images or improving the path generation code?

Changing keystatic.config.ts to path: "src/content/posts/*/" doesn't seem to help, since then the image is still referenced as ![](image.jpeg) when it's in the ./content/ directory

Start with docs/src/content/pages/installation-astro.mdoc and reproduce the image upload in the Astro starter using src/content/posts/first-post.mdoc and keystatic.config.ts. Document a working image setup and confirm that uploaded images are referenced with paths Astro resolves; whether config or path generation changes are needed remains open.