chore: fixed dead links

[cypherpepe]

Why are these changes needed?

Several documentation links in README.md and announcements.md were outdated and returning 404 errors. I updated them to point to the correct current locations in our documentation.

Checks

• I've included any doc changes needed for https://docs.ag2.ai/. See https://docs.ag2.ai/docs/contributor-guide/documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[CLAassistant]

All committers have signed the CLA.