RFR: Updated intro

[Jesper Wilhelmsson]

I re-wrote the intro to talk more about the present and less about the past.

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Commit messages:
 - Updated intro

Changes: https://git.openjdk.java.net/guide/pull/15/files
 Webrev: https://webrevs.openjdk.java.net/guide/15/webrev.00
  Stats: 39 lines in 2 files changed: 32 ins; 4 del; 3 mod
  Patch: https://git.openjdk.java.net/guide/pull/15.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/15/head:pull/15

PR: https://git.openjdk.java.net/guide/pull/15