Add custom 404 page to documentation

[mdapena]

Initial Checks

• The code compiles successfully without any errors or warnings.
• I have personally tested these changes on my local environment.
• I have ensured that relevant documentation has been added or updated.
• I adhere to the project structure and code standards defined in the documentation.
• My code follows the established coding style guidelines.
• I have added tests for the new code (if applicable).
• All existing tests have passed successfully.

Description

This pull request introduces a custom 404 error page to the documentation.