Add customization for non index.md in mkdocs

[cmungall]

In
linkml/linkml#1709

@sujaypatil96 notes

The cookiecutter creates a project directory structure for you which includes an mkdocs.yml file, and the index/landing page is "hardcoded" in the mkdocs.yml file on this line: https://github.com/linkml/linkml-project-cookiecutter/blob/main/%7B%7Bcookiecutter.project_name%7D%7D/mkdocs.yml#L15

At the very least we should make a note on our documentation page for the markdown generator: https://linkml.io/linkml/generators/markdown.html that users will need to change the name of their index file in mkdocs.yml if they've created their schema repos using thecookiecutter.