docs: Replace PLACEHOLDER with local dive output image

[lcampanella98]

Fixes Issue

Fixes #785

Problem

The "Inspect your app" how-to page had a literal PLACEHOLDER where a screenshot of the dive multi-process-app output should be. Readers saw the word "PLACEHOLDER" instead of the image the surrounding text describes.

Solution

I replaced the placeholder with the dive output screenshot. Instead of hotlinking the image from imgur (the existing precedent in this repo), I committed the PNG into the theme's static assets and reference it locally:

• Added themes/buildpacks/static/images/dive-inspect-app.png.
• Referenced it as /images/dive-inspect-app.png, matching how every other diagram in the docs is linked.

This avoids a runtime dependency on imgur, which can prune images that go unviewed for a while. I also gave the image alt text, which the imgur references elsewhere lack.

Testing

• Confirmed the committed PNG is the correct dive multi-process-app screenshot (shows Layers, Layer Details, Image Details, and Current Layer Contents).
• Built the site locally and verified the image renders on the page in place of the old placeholder.

Additional Info

• Screenshot by @gogolok, originally contributed in PLACEHOLDER needs to be populated with dive analysis  #785.
• There are 3 existing references to imgur in this repo rather than referencing local static images. They currently work fine but it's possible imgur could remove them in the future. If requested, I could move those references to static images in a followup to this, just let me know.