Add a Python-based tutorials website generator and deploy workflow

[alexreinking]

Summary

• Replaces the old two-repo, gdb-only shell pipeline (tutorial/figures/generate_output_snippets.sh + halide.github.com/tutorials/gen_tutorials.sh) with a single in-tree Python tool under tutorial/website/, wired in as an opt-in CMake feature (WITH_TUTORIAL_WEBSITE, off by default, requires WITH_TUTORIALS) with its own install component (Halide_Tutorial) and directory knob (Halide_INSTALL_TUTORIALDIR).
• The generator drives either GDB or LLDB in batch mode to capture the output of interesting statements (Func::realize(), print_loop_nest(), etc.) directly from the built lesson binaries, highlights the code with Pygments, and renders a dependency-free static site (no JS, no Bootstrap) with collapsible <details> output blocks, inlined figures, and a nav sidebar that groups multi-part lessons (10, 15, 16, 21) under one header.
• Requires no changes to the lesson .cpp/.sh sources themselves beyond giving the multi-part lessons distinct, descriptive titles (they previously said just "part 1"/"part 2", or in lesson 21's case, were identical) and adding gdb/lldb-friendly compile flags gated behind the new feature.
• Adds .github/workflows/deploy-docs.yml: builds the Doxygen API docs (WITH_DOCS) and the new tutorials website on release and manual dispatch, then deploys both to halide/halide.github.com using the same GitHub App as the existing LLVM-updater workflows. Manual runs default to a dry run (build + diff only, artifact upload instead of a push) and can target a personal fork via an input to rehearse a real push safely.

Test plan

• Built libHalide + all 27 tutorial lesson binaries locally with -DWITH_TUTORIAL_WEBSITE=ON; ran the generator via LLDB and confirmed real captured output (e.g. HL_DEBUG_CODEGEN trace for lesson 3, print_loop_nest/tracing output for lessons 4/5/8/9/18/19).
• Confirmed figure embedding, multi-part nav grouping, and the new shell-script lesson (15's lesson_15_generators_usage.sh) all render correctly.
• cmake --install --component Halide_Tutorial lands files under share/tutorial/.
• ruff check/ruff format and this repo's check_cmake_style.py/check_cmake_file_lists.py all pass; full pre-commit run --all-files-equivalent hooks pass on every commit.
• deploy-docs.yml itself hasn't been run in CI yet -- recommend triggering it once via workflow_dispatch with the default dry_run: true (safe, no push) to sanity-check the artifact before relying on it for a real release.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.04%. Comparing base (8cf05ac) to head (743b5cc).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9347      +/-   ##
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- Coverage   70.06%   70.04%   -0.03%     
==========================================
  Files         258      259       +1     
  Lines       78702    79180     +478     
  Branches    19160    19317     +157     
==========================================
+ Hits        55143    55461     +318     
- Misses      17871    17914      +43     
- Partials     5688     5805     +117     

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