Add jekyll theme as subrepository

[dubreuia]

with makefile that copies the files

[dubreuia]

the theme is hard to find

• https://github.com/pages-themes/cayman (not the same, html tags different, "skip" tag)
• https://github.com/pietromenna/jekyll-cayman-theme (not same head)

[dubreuia]

It's https://github.com/pages-themes/cayman (see https://github.com/pages-themes/cayman/issues/55)