docs: fix README images that do not render on the API reference site

[dfernaro]

Issue # (if applicable)

Closes #34023.

Reason for this change

Three README images are referenced by a repository-relative path. That resolves on GitHub, but the API reference site renders the README at a different base URL, so the browser requests the images from a path that does not exist and they silently render as broken.

Confirmed against the live pages:

Reference	Requested URL on the docs site	naturalWidth
images/ui-screenshot.png	https://docs.aws.amazon.com/cdk/api/v2/docs/images/ui-screenshot.png	0
doc-images/unprocessed-template.png	https://docs.aws.amazon.com/cdk/api/v2/docs/doc-images/unprocessed-template.png	0
doc-images/processed-template.png	https://docs.aws.amazon.com/cdk/api/v2/docs/doc-images/processed-template.png	0

Every other image on those same pages is referenced absolutely and renders correctly, which is the pattern the status badges at the top of every module README already follow.

• https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_synthetics-readme.html
• https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.cloudformation_include-readme.html

Description of changes

Point the three references at raw.githubusercontent.com, which serves them as image/png. The image files themselves are unchanged and stay where they are.

These are the only repository-relative image references left in the repo, so no other README is affected:

$ grep -rnP '!\[[^]]*\]\((?!http)[^)]*\.(png|jpg|jpeg|gif|svg)' --include='*.md' packages/
(no matches)

No code block is touched, so Rosetta is unaffected.

One decision worth flagging: the URLs are pinned to main rather than to a commit SHA. That keeps them consistent with how the READMEs already link to other repository content, and lets the images be updated in place, at the cost of breaking if the files are ever moved or renamed. Happy to pin to a SHA instead if you prefer immutability.

An alternative considered and rejected: removing the images altogether. They carry real explanatory value — the cloudformation-include pair shows the exact "View processed template" checkbox state the surrounding prose depends on — so fixing the reference seemed better than dropping the content. Happy to switch to removal if that is the preferred direction.

Describe any new or updated permissions being added

None. This is a documentation-only change.

Description of how you validated changes

No unit or integration tests, as no code changed.

Validated by hand:

1. Loaded both published documentation pages and inspected every <img>. The three relative references resolve under /cdk/api/v2/docs/ and report naturalWidth === 0; all absolutely-referenced images on the same pages report a non-zero width.
2. Requested each replacement URL directly and confirmed it is served as image/png with the expected dimensions — 1156×315, 2278×536 and 2280×566 respectively.
3. Re-ran the repo-wide grep above to confirm no relative image reference remains.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license