fix: update llms links

[sunggyeol]

Summary

• Updates llms.txt and llms-full.txt links from brittle/moved GitHub docs paths to current public https://adk.dev pages.
• Replaces moved or dead external targets for A2A samples, Arize resources, Model Garden, MCP Toolbox, and missing legacy image assets.
• Updates the build_llms_txt.py generator so future generated docs use public docs URLs, resolve copied docs-relative links, and keep copied README links link-checkable.

Validation

• npx --yes markdown-link-check -q llms.txt
• npx --yes markdown-link-check -q llms-full.txt
• python3 -m py_compile contributing/dev/utils/build_llms_txt.py
• git diff --check
• uv run --with pyink pyink --check contributing/dev/utils/build_llms_txt.py

[rohityan]

Hi @sasha-gitg , PTAL

[rohityan]

Hi @sunggyeol , Thank you for your contribution! We appreciate you taking the time to submit this pull request. Please fix formatting errors by running autoformat.sh

[sunggyeol]

Hi @rohityan, thanks for the review. I ran the repository formatting checks via pre-commit since autoformat.sh does not appear to exist in the current checkout. The formatting fix has been pushed in 0f0f467.