Add a table of contents to DIRECTORY.md

[priya-sundaram-dev]

Describe your change:

Adds a linked Table of Contents of the top-level sections to the top of DIRECTORY.md, so readers can jump straight to a category (Sorts, Graphs, Ciphers, …) instead of scrolling.

This is the auto-generated version of the idea in #13239 — rather than hand-editing DIRECTORY.md (which is overwritten by the directory writer), the ToC is produced by scripts/build_directory_md.py, so it stays in sync automatically. Thanks @cclauss for pointing to the right place.

Details:

• New md_anchor() helper turns a section heading into a GitHub-style anchor slug, with doctests.
• print_directory_md() first emits ## Table of Contents with one * [Section](#anchor) link per top-level section (order preserved), then the existing listing — no lines removed, purely additive.
• DIRECTORY.md regenerated to match.

Closes #13111

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. (Tooling/docs change requested by a maintainer; touches scripts/build_directory_md.py and its generated output DIRECTORY.md.)
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms include at least one URL that points to Wikipedia or another similar explanation. (N/A — build tooling, not an algorithm.)
• If this pull request resolves one issue, please mention that issue number.

[cclauss]

This is great, but can you make the table of contents a numbered list so we can see at a glance how many algorithm folders we have? Also, can you please add a similar link, but without the pound sign, to each section title? When I am scrolling through the sections and I see one that I want to explore, I can click the section title and jump right to the algorithm directory.

[priya-sundaram-dev]

Thanks @cclauss — both done in build_directory_md.py so they stay auto-generated:

1. Numbered ToC — the Table of Contents is now a numbered list (1., 2., …), so the last number is the total count of algorithm folders at a glance (currently 45).
2. Clickable section titles — each top-level heading now links to its directory, e.g. ## [Sorts](sorts), so you can click a section title while scrolling and jump straight to that algorithm folder. The ToC anchor links (#sorts) are unchanged and still resolve, since GitHub builds the heading anchor from the link text.

DIRECTORY.md regenerated; the only content changes are the ToC format and the heading links (no entries added or removed). Doctests pass.

[cclauss]

Nice!!