[valhalla-docs] RFR: Fix wrong link to JEP 401 and add background info links

[Nicolai Parlog]

On Wed, 7 Jan 2026 21:39:09 GMT, Nicolai Parlog <duke at openjdk.org> wrote:

> The link to JEP 401 pointed to 390. Unrelated, I also added a talk from Dan and an article from John to the list of background documents. Maybe they were intentionally left out, so no hard feelings if they're removed. 😉 And the GitHub editor added a trailing newline, I think. 😁

The important change (fixing the JEP link) was made in 7bb7fda, so I'll close this PR.

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PR Comment: https://git.openjdk.org/valhalla-docs/pull/16#issuecomment-3765614228