Bug 1568472 - Add Gitbook hint plugin. r=jdescottes.

The template is used in one of the page, but the
plugin wasn't installed, so Gitbook would fail
generating the doc.

Differential Revision: https://phabricator.services.mozilla.com/D39156

--HG--
extra : moz-landing-system : lando

devtools/docs/about-documentation.md

@@ -13,6 +13,9 @@ npm install -g gitbook-cli
 # Go into the docs directory
 cd /path/to/mozilla-central/devtools/docs/
 
+# Install plugins
+gitbook install
+
 # Generate the docs and start a local server
 gitbook serve
 

devtools/docs/book.json

@@ -0,0 +1,3 @@
+{
+  "plugins": ["hints"]
+}