Update Quickstart re: kramdown default

Update Quickstart documentation to reflect Jekyll’ 2.0 default of
kramdown Markdown engine versus Redcarpet.
Remove Quickstart' Default Markdown Section

Remove default Markdown section from Quickstart documentation page as
it is redundant and unnecessary at this time. #2387
Merge pull request #2387 from jaybe-jekyll/doc_updates
Update history to reflect merge of #2387 [ci skip]
Remove broken link in extras.md, closes #2400
Merge pull request #2401 from jdavis/patch-1
Update history to reflect merge of #2401 [ci skip]
override EXCERPT_ATTRIBUTES_FOR_LIQUID

Allow EXCERPT_ATTRIBUTES_FOR_LIQUID to be overridden by inheriting class. Right now will always reference Jekyll::Post::EXCERPT_ATTRIBUTES_FOR_LIQUID. This is already being used in the codebase for ATTRIBUTES_FOR_LIQUID.
Header links to pages which have a title

Header used to add link to all pages of the site, even those which do not have any title, creating empty anchors on the page like `<a class="page-link" href="/feed.xml"></a>`. Those were non click-able and used space on the page due to margins.

Now only displays pages with a title.
Merge pull request #2410 from XhmikosR/master
Update history to reflect merge of #2410 [ci skip]
Merge pull request #2411 from mathbruyen/patch-1
Update history to reflect merge of #2411 [ci skip]
Merge pull request #2408 from quinn/patch-1
Update history to reflect merge of #2408 [ci skip]