add doc-templates folder for jinja templates

[sujaypatil96]

Addresses issue #10

[hrshdhgd]

Have you tested this locally? If it works then I think this looks good!

[sujaypatil96]

It does work, but @cmungall is concerned that keeping the jinja templates https://github.com/linkml/linkml/tree/main/linkml/generators/docgen bundled with core linkml and the jinja templates here in this repo in sync will be challenging, which makes sense. What do you think?

[cmungall]

let's hold off for now until we have decided on the full lifecycle of the jinja

I think it's OK to make more technical people jump through the hoops of copying the files manually, as they will need technical expertise to edit the local jinja anyway