RFR: Copy images to dist directory on build

[calnan]

Updates to the Makefile to copy across any image files into the build/dist directory so that the images appear when the index.html file is viewed.

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Commit messages:
 - Moved images into separate directory and so no longer need images.conf file
 - minor updates from feedback
 - remove mermaid error file on make clean
 - Merge branch 'openjdk:master' into CopyImages
 - Updates to copy over images to the build/dist directory

Changes: https://git.openjdk.org/guide/pull/111/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=111&range=00
  Stats: 7 lines in 2 files changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/guide/pull/111.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/111/head:pull/111

PR: https://git.openjdk.org/guide/pull/111