RFR: Addition of a causes link type

[Roger Calnan]

There is a proposal for the addition of a causes/caused by link type.  Tied into that there is now a separate section which covers the main link types that we use

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Commit messages:
 - draft of an update of guide to cover the potential addition of a causes link type

Changes: https://git.openjdk.org/guide/pull/139/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=139&range=00
  Stats: 19 lines in 1 file changed: 18 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/guide/pull/139.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/139/head:pull/139

PR: https://git.openjdk.org/guide/pull/139