Localise third-party doc images to site/public/docs-images/

Pulled 15 doc images that were being hot-linked from imgur and from
the legacy github user-images.githubusercontent.com host, converted
each to lossy WebP at quality 80 with libwebp's slowest/best method,
and committed the local copies under site/public/docs-images/. All
ten doc files that referenced them now point at the local /docs-images/
URL instead.

That removes the external dependency (a hot-linked imgur image can
disappear or get throttled at any time, and the GH user-images CDN
deletes assets unattached to live issues) and shrinks total payload —
each image went from 100-500KB PNG to 1-26KB WebP, 256KB total for
all fifteen.

The JetBrains sponsor logo on the credits page is intentionally NOT
localised: it's a sponsor brand asset served from JetBrains' own
brand-resources CDN and the sponsor is the source of truth for it,
not us.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: henderkes