Bump theme to v0.4.8

[svekars]

Bumps pytorch_sphinx_theme2 to v0.4.8 and updates CHANGELOG.

Changes

Version bump to 0.4.8 in setup.py, init.py, and CHANGELOG.md
Fixed init.py version — was stuck at 0.4.6 from a missed bump in the v0.4.7 release

What's in v0.4.8

• Parallel Markdown Generation (PR Parallelize markdown generation with ProcessPoolExecutor #243) — Parallelized HTML-to-markdown conversion using ProcessPoolExecutor for significantly faster llms.txt builds on large doc sets. Includes automatic fallback to sequential processing when multiprocessing is unavailable (e.g., sandboxed environments like Netlify). Added comprehensive parallelism tests covering correctness, determinism, and synthetic-scale workloads.

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