Reorganize docs and switch to ruff

[abrookins]

Summary

Reorganizes documentation and switches linting from black/isort/flake8 to ruff.

Documentation Changes

• Split models.md into two focused pages:

  • models.md - Model definition, fields, validation, saving/loading, configuration
  • querying.md (new) - Comprehensive querying API documentation

• Merged validation.md into models.md - Validation content now lives in the Models page

• Updated migrations.md - Removed references to deprecated migrate command (only om migrate exists now)

• Added make docs target - Start local docs server for preview

Linting Changes

• Replaced black, isort, flake8 with ruff
• Added ruff configuration to pyproject.toml
• Minor formatting fixes from ruff

New querying.md covers:

• All expression operators (==, !=, <, >, %, <<, >>, etc.)
• String methods (startswith, endswith, contains)
• Combining expressions with &, |, ~
• Terminal methods (all(), first(), count(), page())
• Sorting and stable pagination
• Field projection (.values(), .only())
• Bulk operations (.update(), .delete())
• Vector search with KNNExpression
• RedisVL integration
• Raw Redis commands
• Debugging and performance tips