[amber-docs] RFR: Updated index for JDK 18 work in progress

[Gavin Bierman]

Updating the index page

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Commit messages:
 - Updated index for JDK 18 work in progress

Changes: https://git.openjdk.java.net/amber-docs/pull/11/files
 Webrev: https://webrevs.openjdk.java.net/?repo=amber-docs&pr=11&range=00
  Stats: 4 lines in 1 file changed: 2 ins; 1 del; 1 mod
  Patch: https://git.openjdk.java.net/amber-docs/pull/11.diff
  Fetch: git fetch https://git.openjdk.java.net/amber-docs pull/11/head:pull/11

PR: https://git.openjdk.java.net/amber-docs/pull/11