RFR: Links to top

[Jesper Wilhelmsson]

Added links back to the top of the document after each section.
Also did minor cleanups of the text and added an example graph showing a backport hierarchy.

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Commit messages:
 - Links to top

Changes: https://git.openjdk.java.net/guide/pull/72/files
 Webrev: https://webrevs.openjdk.java.net/?repo=guide&pr=72&range=00
  Stats: 78 lines in 1 file changed: 71 ins; 0 del; 7 mod
  Patch: https://git.openjdk.java.net/guide/pull/72.diff
  Fetch: git fetch https://git.openjdk.java.net/guide pull/72/head:pull/72

PR: https://git.openjdk.java.net/guide/pull/72