docs: repair optimal growth and Bansal-Yaron references

[lui01212]

The link-check report identifies three outdated optimal-growth references and one broken Bansal/Yaron publication URL.

• Point the two model references to the current optimal-growth lecture.
• Remove the obsolete “Exercise 2” cross-reference while preserving the capital-stock exercise itself. Exercise 2 on the current page now covers timing the Bellman operator, so replacing its URL alone would point readers to a different exercise.
• Use the publisher's DOI for Bansal and Yaron (2004): 10.1111/j.1540-6261.2004.00670.x.

Closes #369.

Validation: verified the lecture page and the publisher's publication record; checked that fenced code blocks and display equations are unchanged; checked that the bibliography differs only in this URL. git diff --check passes. The lecture build and code execution have not been run. Unrelated redirects in the report are outside this change.

AI assistance was used to prepare this documentation change.

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