chore: add gitignore for assets

see comment in file for more context

src/sphinx_theme_ref/assets/.gitignore

@@ -0,0 +1,20 @@
+# generated by Tiara's HTML Theming Reference Implementation
+#
+# this .gitignore serves the purpose of allowing wrapping projects, that
+# generate this project's assets dynamically, not having to worry about
+# auxiliary debugging assets polution. The file is supposed to be copied by
+# webpack into the output directory, so the globbing patterns shouldn't match
+# the current directory. That's why it's called `gitignore` instead of
+# `.gitignore` so that it isn't actually being picked up by git. webpack should
+# rewrite the basename.
+#
+# Depending on your level of paranoia towards breaking changes in dependencies
+# with no access to working versions, you can either choose to ignore the entire
+# directory and generate HTML theme assets just-in-time, or commit them as
+# static assets. I'm running registry proxies as pull-through mirrors locally,
+# that I back up regularly, to keep my paranoia at bay. That way, I can.just
+# gitignore the directory and generate all assets dynamically.
+*
+!/.gitignore
+*.ts
+*.map