refactor: integrate mkdocs asset doc generation into class_generator.py

- Replace render_sphinx_docs() with render_mkdocs_docs() in Generator
  class; now generates docs/api/assets/*.md grouped pages (26 categories)
  instead of individual per-class .rst files
- Uses AssetInfo.module_name and is_core_asset to determine the correct
  module path for each ::: directive, mirroring __PYATLAN_ASSETS__
- Remove sphinx_asset.jinja2 and sphinx_asset_index.jinja2 templates
  (no longer needed)
- Remove standalone generate_asset_docs.py (logic now in generator)
- Rename SPHINX_DIR constant to MKDOCS_ASSETS_DIR pointing at
  docs/api/assets/

Running class_generator.py will now regenerate docs/api/assets/ pages
automatically alongside the Python asset source files.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Aryamanz29