In an effort to share a learning resource; is it appropriate to post links to external sites? What is the right way to post a link to a learning resource?
Please don't. I don't think anybody is waiting for a partial implementation of SHA-256 in VBscript or Excel formulas. This is a Q/A site. If you really want to share anything use a blog or - for smaller tidbits - Twitter.
That said, as SEJPM already indicated, the side channel is kind of a catch all, chat along, share anything but remember that anything you post there may be stay visible for a long time.
Links in questions and answers are highly appreciated especially when it comes to references, but please consider that link only answers are not accepted. Both questions and answers should be self-contained, regardless if the link contents.
[...]is it appropriate to post links to external sites?
If the links helps to more easily understand your question or your answer, or allows the interested reader to find further information, then yes, this is very welcome.
If you don't formulate your post as a question or an answer and essentially just put a link to a product or a service in there (as your original post before the edit did), the I think this can be considered "spam" (and will be flagged as this), additionally to such questions being closed because they don't ask anything, additionally to such questions being down-voted (same applies to answers).
What is the right way to post a link to a learning resource?
Our chat, The Side Channel is the appropriate place to place such a link as long as it's relevant to cryptography and doesn't (aggressively) advertise a (fishy / paid) service.