w.org: Handbook Plugin: Support Jetpack Markdown

[kraftbj]

In plugins/handbook/handbook.php line 242, we need to add wpcom-markdown to the CPT supports field so Jetpack's Markdown feature will render the content.

Originally reported in ​https://github.com/Automattic/jetpack/issues/6143

[kraftbj]

based in the handbook SVN external

[pento]

In 4753:

Handbook: Add wpcom-markdown support to the handbook post type.

This ensures Jetpack will render Markdown content (particularly in the WP-CLI handbook) correctly.

Props kraftbj.
Fixes #2417.