remove docs on installing pygments

This is no longer needed as stated by @parkr in #1021,
as jekyll now comes bundled with pygments.rb.

Fixes #1021.
Update gemspec for new page-based docs.
Merge pull request #1079 from maul-esel/docs-remove-pygments

Remove docs on installing pygments
Update history to reflect merge of #1079
Turn off relative permalinks on Jekyll docs.