[amber-docs] RFR: Update and restructure JEP list

[Nicolai Parlog]

Update and restructure JEP list

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Commit messages:
 - Add JEP 458
 - Update and restructure JEP list

Changes: https://git.openjdk.org/amber-docs/pull/25/files
  Webrev: https://webrevs.openjdk.org/?repo=amber-docs&pr=25&range=00
  Stats: 22 lines in 1 file changed: 4 ins; 16 del; 2 mod
  Patch: https://git.openjdk.org/amber-docs/pull/25.diff
  Fetch: git fetch https://git.openjdk.org/amber-docs.git pull/25/head:pull/25

PR: https://git.openjdk.org/amber-docs/pull/25