Add a table of contents to DIRECTORY.md (#15198)

* Add a table of contents to DIRECTORY.md

Generate a linked table of contents of the top-level sections at the top
of DIRECTORY.md so readers can jump straight to a category, as suggested
in #13239 / #13111. The list is built in scripts/build_directory_md.py
(with a new md_anchor helper + doctests) so DIRECTORY.md stays fully
auto-generated.

* Address review: numbered ToC + link section headings to their directories

- Table of Contents is now a numbered list, so the final number shows the
  total count of algorithm folders at a glance.
- Each top-level section heading links to its algorithm directory (e.g.
  ## [Sorts](sorts)), so clicking a section title jumps straight to the folder.

Author: priya-sundaram-dev