FEAT: Folder structure for source and build directories

[AakashGfude]

The folder structure needed for source and build directories while creating a project with jupinx. It can be either hard-coded or configurable, and can have different folders or only one base folder etc.

[mmcky]

Let's run with hard coded (or templated) with a fixed directory structure:

• [ ] source for source rst, md files (@jstac do you want to force others into the rst directory, as this is markup specific. We have adopted that structure in our own projects but I would recommend we default to having files in source to cater for different input types)
• [ ] source/_static for static assets and files.
• [ ] _build I think the convention of using _ for _build is useful in indicating it is not permanent
• [ ] conf.py at the root level
• [ ] Makefile at the root level

more controversial:

• [ ] theme folder at the root level (@AakashGfude should this include templates?). This is typical for html themes for look and feel. I tend to think we should have a theme and templates folder (and adopt this structure for our own repos as well)
• [ ] templates folder (?)