copy over existing documentation

[skatkov]

Rails has a guides pages that are defined as markdown files, yard-markdown should copy this over as well.

[skatkov]

As example, rails has a guides/source folder. I may have to solve this outside of this gem, though.

https://github.com/rails/rails/tree/main/guides/source

Rails also has a and readme.md file:
https://github.com/rails/rails/blob/main/README.md

I'm not usre, that using readme as index is always reasonable. So I need an ability to define any "index" file? maybe generate table of contents as default as possible option.