Add usage section
This MR:
- adds usage explanation about KDocTools
- mentions the best tool to automatically format our DocBook files I could find (xmlformat)
- mentions using KHelpCenter to view the converted files¹
- mentions what currently doesn't work²
[1] Using KHelpCenter ensures the content will display correctly, the generated HTML files do not seem to show up properly on Firefox.
[2] I did not manage to make kde-nochunk.xsl behave as intended at all, the kde-web.xsl stylesheet did not seem to provide a full HTML file (missing images for instance), and there is no kde-man.xsl stylesheet.