Give a build warning if a layout does not exist

Throw a warning if a non-"none" layout is specified but the corresponding
layout file does not exist.
Do the same for documents
Move layout validity logic to separate method
Add path of file requesting nonexistent layout
Fix up errors in CI build
Exclude excerpts from layout checks
Use `is_a?` instead of `class`
Update outdated "Extras" page and remove duplicate documentation
Fix URL

Copy & paste error introduced a `.dev` local domain; change
to a relative path instead
Remove resolved RedCloth issue from troubleshooting
Added kramdown-with-pygments plugin

This plugin enables Pygments syntax highlighting for Kramdown-parsed fenced code blocks
Update the history I guess.
Merge pull request #2623 from mvdbos/master
Update history to reflect merge of #2623 [ci skip]
Merge pull request #2622 from alfredxing/fix-markdown-docs
Update history to reflect merge of #2622 [ci skip]
Remove unnecessary Jekyll::Tags::IncludeTag#blank? method
Merge pull request #2625 from fw42/remove_unnecessary_blank_method
Update history to reflect merge of #2625 [ci skip]
Add jekyll-hook to deployment methods.

Port of #2617 to the master branch.