markdownfy_view is unauthenticated - Please consider changing it

Currently the installation document/README just says add URLs to your Django. No issues with this part. What is not specified here is that the URL `/martor/markdownify/` now becomes "public". By itself, it is not a problem. There maybe many "mitigating" circumstances such as ALBs that don't allow `/martor/` URLs and such. But, if someone were to just directly deploy this without reading the code, they have unintentionally exposed an _unauthenticated_ URL.

Either we should call it out explicitly in the README, or better yet, there should be no reason for the function `markdownfy_view` to not have the `login_required` decorator which all the other functions have.