Co-Authored-By: Peter Mosses <18308236+pdmosses@users.noreply.github.com>

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`.

Added documentation of default nav order.

This reverts commit 8ac545a5.

This reverts commit 89f79138.

This reverts commit 8ac545a5.

The current default is that when a grandparent is selected, the links to its
children *and all its grandchildren* are displayed in the navigation menu.
Setting `grandchildren_branch: true` in `_config.yml` delays the display of links
to grandchildren until their parent is selected.

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.