docs: group all 770 open PRs by algorithm directory in hacktober_2026_prep.md

[priya-sundaram-dev]

As requested by @cclauss on #15081, adds a new section at the bottom of docs/hacktober_2026_prep.md grouping all 770 open pull requests by the algorithm directory they modify, in the requested format:

Total open pull requests: 770

audio_filters:
1. [ ] #14979
backtracking:
1. [ ] #10102
...

How it was built: I examined the changed files of every open PR (GET /pulls/{n}/files) and bucketed each by its top-level algorithm directory. Each PR appears in exactly one grouping (verified: 770 assigned, 0 duplicates).

Special groupings (as you specified):

• Modifies multiple algorithm directories — 53 PRs touching more than one top-level algorithm dir.
• Modifies no algorithm directories — 36 PRs touching only non-algorithm paths (.github/, scripts/, docs/, root files).
• Modifies DIRECTORY.md — 1 PR touching only DIRECTORY.md.

Largest buckets: maths (105), sorts (67), machine_learning (58), data_structures (57), strings (45), searches (33).

The audio_filters group has a single PR (#14979) — happy to start there with Approve / Request changes once scripts/find_git_conflicts.sh (#15181) is sorted.

Docs-only change; nothing under an algorithm directory is touched.

[algorithms-keeper]

Closing this pull request as invalid

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