docs: add Hacktoberfest 2026 open-PR cleanup tracker (docs/hacktober_2026_prep.md)

[priya-sundaram-dev]

Describe your change:

Adds docs/hacktober_2026_prep.md, the tracker for the pre-Hacktoberfest open-PR cleanup requested in #15081.

It contains:

• A short intro explaining the purpose (clearing the queue before the October influx).
• All 918 currently-open pull requests as a Markdown checklist in the N. [ ] #pr_id <labels> notation, e.g. 1. [ ] #15158 enhancement, awaiting reviews (PRs with no labels are shown as no labels).
• A Closed totals table at the bottom, one row per scripts/close_pull_requests_with_*.sh job plus a grand total, all starting at 0.

Workflow once this is merged: after each close_pull_requests_with_*.sh run, I'll flip the affected [ ] -> [x], replace the labels with closed by <script name>, and update the totals table — as coordinated in #15081.

@cclauss — snapshot taken 2026-09-02. If you'd rather I sort the list a different way (e.g. oldest-first to prioritize stale PRs) or group by label, say the word and I'll regenerate.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• Documentation change?

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. To ease review, please open separate PRs for separate algorithms.
• 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.
• If this pull request resolves one or more open issues then the description above includes the issue number(s) with a closing keyword: "Fixes #ISSUE-NUMBER".

[cclauss]

Nice! No need to sort or add new items to the list or remove items from the list.