Reinstated the collapsible TOC at the top, to support the reference to it right at the end of the file. (The `TOC` feature can only be used once per page, so this is the only way of illustrating the rendering of the collapsible TOC in the docs.)

Clarifications

Adjusted the documentation to explain how mixtures of numbers and strings are treated by `nav_order`.

Makes the TOC at the top of the page collapsible.
Adds an explanation of the markup used to produce a collapsible TOC.
Adds a comment about the possibility of using an unordered list for the TOC,
and about using TOC only once per page.

closes #266

Deleted a spurious line from a previous version.

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.