.gitignore: add auto-generated jekyll paths

* .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] jekyll/jekyll#8648
[3] jekyll/jekyll#8646

Author: kmk3

.gitignore

@@ -1,2 +1,5 @@
-_site
+_site/
+.jekyll-cache/
+.jekyll-metadata
+
 .DS_Store