Remove legacy Jekyll files

Gemfile

Gemfile.lock

_config.yml

_includes/footer.html

_includes/head.html

_includes/header.html

_includes/icon-github.html

_includes/icon-github.svg

_includes/icon-gitlab.html

_includes/icon-gitlab.svg

_includes/icon-twitter.html

_includes/icon-twitter.svg

_includes/image.html

_layouts/default.html

_layouts/page.html

_layouts/post.html

_posts/.gitkeep

_sass/_base.scss

_sass/_layout.scss

_sass/_syntax-highlighting.scss