List real-world examples of Echidna/Manticore integration

- https://blog.trailofbits.com/2020/08/17/using-echidna-to-test-a-smart-contract-library/
- https://github.com/search?q=%22function+echidna_%22&type=Code
- Some of our public reports: https://github.com/trailofbits/publications

Review the linked Echidna blog post, code search results, and Trail of Bits public reports for real-world examples of Echidna/Manticore integration. Add a sourced list to the relevant building-secure-contracts training material; done means the examples are documented with links and no longer limited to the current references.

Written by the indexing model from the issue text.