Update .gitignore (just-the-docs#34)

See: just-the-docs/just-the-docs#1327

.gitignore

@@ -1,5 +1,10 @@
-# Copied from https://github.com/github/gitignore/blob/main/Jekyll.gitignore
-# Ignore metadata generated by Jekyll
+# Not sure what a .gitignore is?
+# See: https://git-scm.com/docs/gitignore
+
+# These are directly copied from Jekyll's first-party docs on `.gitignore` files:
+# https://jekyllrb.com/tutorials/using-jekyll-with-bundler/#commit-to-source-control
+
+# Ignore the default location of the built site, and caches and metadata generated by Jekyll
 _site/
 .sass-cache/
 .jekyll-cache/