Manually add alt-text to all images for which it is currently missing

Created by: seancolsen

Migrated from: https://issues.civicrm.org/jira/browse/CRM-18228

Lots of images in the User Guide don't have alt text. We should add alt text to these images to improve accessibility.

Here's a command which finds lots of these grep -Pir '\!\[(image)+\]\([^)]*\)' (currently 251 results)