Added a configuration option to determine whether the sort order is case-sensitive.
The default is case-insensitive.
To test:
- open `/just-the-docs/docs/utilities/` in the browser,
and check that the navigation links in `Utilities` are sorted alphabetically;
- in `docs/utilities/layout.md', change the preamble to `title: layout`,
and check that the links in `Utilities` are still sorted alphabetically;
- add `nav_sort: case_sensitive` in the configuration file,
and check that the link to `layout` is now listed last under `Utilities`.
Using `index.md` as name for the parent page within a section allows lazy developers
to skip defining `permalink`. This is already working and this PR just updates the documentation.
