RFR: More prominent h1 headers

[Jesper Wilhelmsson]

Making it easier to see h1 headers to have some visual separation between sections.

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Commit messages:
 - More prominent h1 headers

Changes: https://git.openjdk.java.net/guide/pull/29/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=29&range=00
  Stats: 13 lines in 3 files changed: 12 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/guide/pull/29.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/29/head:pull/29

PR: https://git.openjdk.java.net/guide/pull/29