Reorganizing material to put page source in sub-directory:

* Add `.nojekyll` to stop GitHub trying to compile.
* Get rid of Jekyll `_config.yml`.
* Create placeholder `css` and `img` directories.
* Create `pages` directory with source for web pages.
* Move all Markdown files into `pages`.
* Move `tools` into `pages` (because that's where people will mostly work).
* Move `_includes` and `_layouts` into `pages` (because that's where we'll compile).

_config.yml

-markdown        : kramdown
-kramdown        :
-  entity_output : as_input # so that — is output as —
-  smart_quotes  : [39, 39, 34, 34] # decimal for left/right single/double quotes
-exclude         : [tools]
-contact         : "admin@software-carpentry.org"
-swc_site        : "http://software-carpentry.org"

css/swc.css

+h1 {
+     color : mediumblue;
+}