Add docs site to /docs directory on master branch

[anselmbradford]

Fixes https://github.com/cfpb/capital-framework/issues/597

This is a first step in getting rid of the gh-pages branch hosted docs site, which is pain point for testing things and keeping the docs in sync with the changes in the codebase. This PR creates a /docs directory on the master branch and adds initial gulp tasks for handling the docs assets.

This PR adds a /docs directory with jekyll template content largely from gh-pages, with the following differences:

• Replaces homepage icons with SVGs.
• Fixes issue where homepage images had their alt attributes erroneously named class.
• Does not include /components/grunt-tasks.md.
• Does not include src/static/css/cf-enhancements.less
• Does not include src/static/css/syntax.less (appears to be redundant to code.less).
• Does not include src/static/gulpfile.js
• Does not include src/static/js/main.js and instead uses compiled capital-framework.min.js, though it probably should use a main.js script in the future.

Additions

• Adds /docs directory with jekyll template content largely from gh-pages.
• Adds gulp docs task for copying documentation assets.
• Adds gulp styles:docs task for compiling documentation less files.

Testing

1. gulp docs should pass.

If you want to run this locally you can follow the instructions on https://help.github.com/en/articles/setting-up-your-github-pages-site-locally-with-jekyll and run jekyll inside the /docs directory. However, you have to remove {{ site.github.url }} from the templates (default.html and home.html) to get the styles to apply.

Notes

• I was unclear if _layouts/post.html was used anywhere, but left it for now.

Todos

• To actually host the docs this way we should probably merge this PR and then adjust the settings to point to it. See https://help.github.com/en/articles/configuring-a-publishing-source-for-github-pages#publishing-your-github-pages-site-from-a-docs-folder-on-your-master-branch

[Scotchester]

• Replaces homepage icons with SVGs.

Were you working off an old version of gh-pages? I did this a little while ago. (including the external link icon in the footer)

• Fixes issue where homepage images had their alt attributes erroneously named class.

It would be better to just empty out these alt attributes, since the images do not have any content value to screen reader users.

[Scotchester]

Let's give it a whirl!

[Scotchester]

One other TODO we should state explicitly: If this works as expected, we'll need to update the contributing doc.