feat: add show_footer config option to hide footer

[FernandoCelmer]

Description

Issue: #28

• mkdocs_dracula_theme/mkdocs_theme.yml: added show_footer: true as a default config key
• mkdocs_dracula_theme/base.html: wrapped the footer include block with {% if config.theme.show_footer %} conditional

Motivation and Context

Users who want to hide the "Made with Dracula Theme for MkDocs" footer had no native way to do so. Setting show_footer: false in mkdocs.yml now suppresses it cleanly without requiring a custom_dir override.

Closes #28

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation updated (documentation was updated and published)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules