Allow REST-API for post-type `handbook` to export contents for translation

[miyauchi]

We are planning to translate contents for the handbook.
Can you activate REST-API for post-type handbook?

My plan of workflow to translate is like following.

1. Export handbook contents via REST-API.
2. Convert contents to markdown format.
3. Translate markdown files on GitHub
4. Publish as gh-pages or import them into WordPress.
5. If original handbook contents will be changed, get the information via REST-API.

Tracking changes for original pages is big problem when we try to translate it, but I guess we can solve this problem by REST-API.

[miyauchi]

Add line 'show_in_rest' => true, .

[obenland]

In 7345:

Handbook: Show handbook posts in REST API.

Props miyauchi.
Fixes #3679.