docs: correct default ia.ini location

[bkjoh]

Summary

Updates documentation for the default ia.ini config path.

Changes

• Document ~/.config/internetarchive/ia.ini as the default path when XDG_CONFIG_HOME is unset or invalid.
• Document $XDG_CONFIG_HOME/internetarchive/ia.ini when XDG_CONFIG_HOME is set to an absolute path.
• Mention the Windows example path.
• Clarify that ~/.config/ia.ini and ~/.ia remain legacy fallback paths.
• Update Python usage examples that referenced ~/.config/ia.ini.
• Update the get_config docstring so the config_file parameter refers to the path selected by parse_config_file.

Validation

• Not run locally. Documentation-only change.

Closes #765.

[jjjake]

Thank you @bkjoh! Sorry for the delay in getting this merged, I appreciate the contribution.