Add doc for WordPress shortcodes in extensions

Created by: seancolsen

In https://github.com/civicrm/civicrm-wordpress/pull/112 @christianwach added functionality which allows CiviCRM extensions to interact with the WordPress shortcode. @kcristiano @JoeMurray @colemanw and @monishdeb also looked at that PR. Monish recommended that we document this change somewhere and asked me to help find a place to document it. I agree that it should be documented and this PR represents my recommendation for where to document it.

However, I don't understand much about WordPress shortcodes (or the associated civicrm-wordpress PR). I just copy-pasted some content from the PR comments to make this documentation. @christianwach would you be willing to expand on this documentation a bit? Or do you think it's sufficient? You can build on top of my work here by either (a) submitting a PR to my dev-docs fork, or (b) creating a new PR on dev-docs with these changes as the base, or (c) suggest changes for me to make in this PR (if they're simple).

@seanmadsen I think it looks good. The new functionality is only intended for Extension developers - who are presumably technically literate :-)

Created by: seancolsen

Cool. Merging now based on approval from @christianwach. Thanks!