.gitignore: add auto-generated jekyll paths

[kmk3]

• .jekyll-cache/: created when running e.g: jekyll serve [1][2]
• .jekyll-metadata: created when running e.g.: jekyll serve -I [1][3]

[1] https://jekyllrb.com/docs/structure/ [2] https://github.com/jekyll/jekyll/pull/8648 [3] https://github.com/jekyll/jekyll/pull/8646