docs: add 18-week HackMD weekly teaching guide and link it into docs/courses

[willismax]

Motivation

• Provide a version-controlled master HackMD lecture/weekly guide to address thin weekly teaching materials and make instructor-facing content reusable and auditable.
• Align transient HackMD announcements with canonical repository artifacts (Notebooks, example code and assignment templates) to avoid duplicate sources of truth.
• Give instructors a turnkey 18-week syllabus (objectives, in-class flow, tasks, templates and rubrics) for the evening Python/API course so weekly planning is immediately usable.

Description

• Add docs/HACKMD_WEEKLY_GUIDE.md, a complete 18-week teaching guide with per-week learning goals, pre-class tasks, 150-minute in-class flow, guided exercises, exit tickets, assessment rubric, a weekly submission template, and an instructor/TA quick-checklist.
• Update courses/evening/README.md to reference the new weekly guide and include a short note about HackMD vs GitHub responsibilities.
• Update docs/README.md and the top-level README.md to include links to the new guide so it appears in the docs index and project landing page.
• Ensure internal relative links in the new document point to repository modules (e.g. 01.Intro-Python, 03.Request, 05.Flask) so instructors can jump to canonical notebooks and code examples.

Testing

• Pre-commit diff/check passed without reported markdown/diff issues.
• Internal link validation: executed a Python script that resolved all changed-file relative links and returned Broken local links: [], indicating no broken local links.
• Structure check: regex scan confirmed ## 第 1 through ## 第 18 headings are present in docs/HACKMD_WEEKLY_GUIDE.md.
• Staged-diff check and index verification passed and repository is in a clean working state in this environment; GitHub CLI authentication was not configured here so a remote Pull Request could not be opened from this runner.

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Codex Task