Convert tabs to spaces in docbook
Tabs in the docbook often cause contributor's editors to introduce
accidental whitespace changes. It was agreed to change to spaces like in
most other files.
This also corrects a problem where there were tabs inside the content.
Thanks to @gregormi for the initial version of the patch.
Test Plan: checkXML5 index.docbook
Reviewed By: Spectacle, ngraham
Subscribers: gregormi, ltoscano, yurchor, rkflx, lueck, ngraham
Differential Revision: https://phabricator.kde.org/D10529