RFR: 8295205: Add jcheck whitespace checking for markdown files

[Magnus Ihse Bursie]

Markdown files are basically source code for documentation. It should have the same whitespace checks as all other source code, so we don't get spurious trailing whitespace changes.

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Commit messages:
 - 8295205: Add jcheck whitespace checking for markdown files

Changes: https://git.openjdk.org/jdk/pull/10671/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=10671&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295205
  Stats: 34 lines in 9 files changed: 0 ins; 0 del; 34 mod
  Patch: https://git.openjdk.org/jdk/pull/10671.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/10671/head:pull/10671

PR: https://git.openjdk.org/jdk/pull/10671