Enable installation as a gem-based theme

[tnlx]

Hi!

Thank you for the clean and very balanced theme!

As title,

• I'm moving stylesheets to /assets because /css is not a recognized directory for gem-based theme.

With gem-based themes, some of the site’s directories (such as the assets, _data, _layouts, _includes, and _sass directories) are stored in the theme’s gem, hidden from your immediate view. Yet all of the necessary directories will be read and processed during Jekyll’s build process.

https://jekyllrb.com/docs/themes/#understanding-gem-based-themes

• Move the footer text to _config.yml to make it customizable when the theme is installed this way.

Possible usage after merging

# Gemfile
gem 'resume-template', git: 'git@github.com:jglovier/resume-template', ref: '...'
#https://bundler.io/guides/git.html

# _config.yml
theme: resume-template

resume_footer:
  - Paragraph 1
  - Paragraph 2