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

[Nicolai Parlog]

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. 😁

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Commit messages:
 - Fix wrong link to JEP 401 and add background info links

Changes: https://git.openjdk.org/valhalla-docs/pull/16/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla-docs&pr=16&range=00
  Stats: 8 lines in 1 file changed: 6 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/valhalla-docs/pull/16.diff
  Fetch: git fetch https://git.openjdk.org/valhalla-docs.git pull/16/head:pull/16

PR: https://git.openjdk.org/valhalla-docs/pull/16