RFR: New section Making a change

[Jesper Wilhelmsson]

The old section "Fixing a bug" was mostly outdated so it has been replaced with a section "Making a change" that holds content relevant to working with the source code and making changes. The still relevant parts of Fixing a bug has been moved to the appropriate places.

Another change that followed from this was the new section "Responsibilities of a Reviewer".

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Commit messages:
 - Cleaned up
 - Second draft
 - First draft

Changes: https://git.openjdk.org/guide/pull/131/files
  Webrev: https://webrevs.openjdk.org/?repo=guide&pr=131&range=00
  Stats: 208 lines in 6 files changed: 84 ins; 121 del; 3 mod
  Patch: https://git.openjdk.org/guide/pull/131.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/131/head:pull/131

PR: https://git.openjdk.org/guide/pull/131