create folder to house jinja templates

[sujaypatil96]

Create a folder called doc-templates in the src folder to house the base jinja templates that the first pass project created by cookiecutter can depend on.

The folder will contain all the files from here: https://github.com/linkml/linkml/tree/main/linkml/generators/docgen

The reason why this is useful is because users would no longer have to go to the above location to fish out starter jinja templates to begin modifying the style and content of their web documentation pages.

[turbomam]

Should this solution also include providing a Makefile rule for generating documentation pages with --template-directory doc-templates

would need a different name from gendoc

[sujaypatil96]

Decide if this is still the best way forward. Close it out if not.

Caveats:

• We can't have an empty doc-templates folder (on GitHub)
• Shouldn't confuse users/developers by exposing more files in src