Missing Documentation

[araffin]

See #1

• Custom Policies (complete example)
• Migration guide (Migration Guide #123 )
• get/set parameters examples (see Get/set parameters and review of saving and loading #138 )
• create eval env
• save/loading of policy alone
• save/load replay buffer
• Clean up type hints from docstrings (see Update docs (custom policy, type hints) #167)
• Fix autodoc for policies (cf TQC sb3 contrib)
• Add results section to algorithms