Track new rst files in git

[damonbayer]

          Intentially. At some point, we only wanted to keep track of `index.rst`, but when new tutorials are added, `git status` was not reporting new `rst` files generated by `.pre-commit-rst-placeholder.sh`.

Now that I think of it, perhaps we could modify the script to also add newly generated rst files via git add?

Originally posted by @gvegayon in #158 (comment)