RFR: Cleaned up internal links and added a note-style

[Jesper Wilhelmsson]

Pandoc doesn't require internal links to have the second "link"-part of the markdown link, so removing that throughout the guide to make it slightly easier to read the source.

Added a new note-style to clean up the source slightly more, and enable a consistent style for notes.

Added comment about not closing JBS issues with open sub-tasks.

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Commit messages:
 - Cleaned up internal links and added a note style

Changes: https://git.openjdk.org/guide/pull/110/files
 Webrev: https://webrevs.openjdk.org/?repo=guide&pr=110&range=00
  Stats: 109 lines in 14 files changed: 19 ins; 23 del; 67 mod
  Patch: https://git.openjdk.org/guide/pull/110.diff
  Fetch: git fetch https://git.openjdk.org/guide.git pull/110/head:pull/110

PR: https://git.openjdk.org/guide/pull/110