Update .gitignore (#1327)

This PR moves over the changes from https://github.com/just-the-docs/just-the-docs/pull/1112, namely making the `.gitignore` more descriptive and bringing it to parity with the recommendations from Jekyll.

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Co-authored-by: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

.gitignore

@@ -1,8 +1,30 @@
+# Not sure what a .gitignore is?
+# See: https://git-scm.com/docs/gitignore
+
+# The first files 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/
+.jekyll-metadata
+
+# Ignore folders generated by Bundler
+.bundle/
+vendor/
+
+# These next files are used by Just the Docs developers. They are not necessary for end users of the theme, only developers.
+
+# We use Stylelint and Prettier, JavaScript tools, to lint and format our own code.
+# We use Node.js as our runtime, so we ignore node_modules
+node_modules
+
+# .DS_Store is a macOS-only metadata file about directories. Convention is to not commit them.
+# See: https://en.wikipedia.org/wiki/.DS_Store
+.DS_Store
+
+# These are legacy globs that typically target Ruby theme developers. We may change these at a later date.
 *.gem
 .bundle
 .ruby-version
-.jekyll-cache
-.sass-cache
-_site
-node_modules
-.DS_Store