[docs] Render Python-only tutorial lessons (lesson_15_runtime) in the docs

[derek-gerstmann]

The Sphinx tutorial generator discovered lessons from C++/shell sources and attached a Python translation only by matching stems, so the one Python-only lesson -- lesson_15_runtime, covering the standalone halide.runtime module -- was never rendered. Teach discover_lessons() to also pick up lesson_*.py files with no C++ counterpart and render them as Python-only pages, register lesson_15_runtime as a tutorial source in doc/CMakeLists.txt so a page is generated for it, and complete its title comment to match the lesson N: Title convention.

Checklist

• Tests added or updated (not required for docs, CI config, or typo fixes)
• Documentation updated (if public API changed)
• Python bindings updated (if public API changed)
• Benchmarks are included here if the change is intended to affect performance.
• Commits include AI attribution where applicable (see Code of Conduct)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.03%. Comparing base (36c47dc) to head (f17c7a2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9408      +/-   ##
==========================================
- Coverage   70.17%   70.03%   -0.15%     
==========================================
  Files         261      261              
  Lines       79396    79396              
  Branches    19358    19358              
==========================================
- Hits        55717    55604     -113     
- Misses      17896    17915      +19     
- Partials     5783     5877      +94     

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