HTML markup section + Intro

[kba]

• Rephrase introduction
• Extended abstract
• Use setext+id headings for level 1/2 headings
• Reduce the number of sections by turning format constants to definitions in definition lists
• Use high-level concept as section title

  32, #46

[zuphilip]

I still see that the link on the code in the beginning is not rendered:

https://rawgit.com/kba/hocr-spec/f1f97e8399b853ee37697f3c9f9b1ebdfbbe6f22/1.2/index.html#introduction

The remaining thins looks fine now.