docs(templating): explain which image formats a deliverable can carry

[yacinekahlerras-turbodocx]

Description

Only PNG and JPEG can be stored inside a generated document or presentation. Any other
format is replaced in the output with a short message:

[Image not supported: SVG — use PNG or JPEG (diagram.svg)]

That message was documented nowhere, so someone who met it in a deliverable had nothing to
search for.

What this adds

A new troubleshooting section, "Images Must Be PNG or JPEG", covering:

• the message itself, and how to read it (format · what to use instead · which file)
• the full list of formats that get replaced: SVG, GIF, WebP, BMP, TIFF, HEIC, AVIF, ICO
• the part that actually confuses people — the rich text editor previews SVG, GIF and WebP
  perfectly well, because a browser can display formats a Word or PowerPoint file cannot store.
  An image can look correct while you're editing and still be replaced in the finished
  deliverable. Called out in a :::caution so it isn't missed.
• how to fix it, including the 2× export tip for logos and diagrams saved as SVG

Also adds it to Quick Troubleshooting Steps and to the page keywords, so searching the
text the product prints actually lands here.

Why the troubleshooting page

"Why did my image turn into text?" is a troubleshooting question, and the page already collects
the other "your variable looks fine but isn't" traps — naming rules, own-line placement,
paragraph markers. This is the same shape of problem, so it belongs beside them rather than in a
new page nobody would find.

Notes

• Base is develop per the repo's branching rule.
• Docs-only change — no code, no build config.