RFR: 8332545: Fix handling of HTML5 entities in Markdown comments

[Jonathan Gibbons]

Please review a small fix to address a crash when handling HTML5 entities in a Markdown doc comment.

The path for the `entities.txt` (originally `entities.properties`) was not correctly imported when importing the latest version of `commonmark-java`. And, the makefiles need to be updated to include the `.txt` file in the image. (The interim module for the interim javadoc already had this fix.)

A simple new test is provided, containing entities that previously caused `javadoc` to crash.

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Commit messages:
 - JDK-8332545: Fix handling of HTML5 entities in Markdown comments

Changes: https://git.openjdk.org/jdk/pull/19317/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19317&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332545
  Stats: 79 lines in 3 files changed: 77 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19317.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19317/head:pull/19317

PR: https://git.openjdk.org/jdk/pull/19317