Add customtag example block

[kdmccormick]

The CustomTagBlock is an interesting little feature of OLX that is undocumented other than in edx-platform docstrings. Still, we don't want to break it without a DEPR.

It would be good to have an instance of the <customtag> block, as well as one instance of a TranslateCustomTagBlock such as <book>.

In order to work, these blocks need OLX-authored template files, which cannot be added via Studio. Once the template files exist, the blocks can be added and edited in Studio. Here's some more context, with a link to sample OLX: openedx/edx-platform#35829 (comment)