Made case-insenstive sorting the default

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`.