Convert tabs to spaces in docbook
Summary:
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
Reviewers: Spectacle, Documentation, ngraham, gregormi
Reviewed By: Spectacle, ngraham
Subscribers: gregormi, ltoscano, yurchor, rkflx, lueck, ngraham
Differential Revision: https://phabricator.kde.org/D10529