chore(docs): remove generated site output and bump version to 0.31.2

- Remove generated documentation/site build artifacts (static HTML, search
  index, objects.inv, mermaid init, sitemap and compressed sitemap).
  This cleans up huge compiled docs under docs/site that should not be
  tracked (examples, quickstart, installation, contributing, api pages,
  search index, mermaid loader, etc.).
- Bump package version from 0.31.1 to 0.31.2 in pyproject.toml.

Rationale: keep the repository free of built/static doc output and update
project version for the next release.

Author: legout