Add serve command to start CORS enabled Flask server

[hemanth]

Add a serve command to start a CORS-enabled Flask server for file conversion.

• New Flask Server: Add src/markitdown/server.py to define a Flask server with CORS enabled and a route to convert files to markdown.

• Dependencies: Update pyproject.toml to include flask and flask-cors as dependencies and add the serve command to the [project.scripts] section.

• Documentation: Update README.md with instructions on how to use the serve command.

• Tests: Add tests in tests/test_markitdown.py to verify the functionality of the serve command, including handling both URL and file POST data.

In case we decide not to have serve command we have markdown-converter.

[hemanth]

@microsoft-github-policy-service agree

[hemanth]

@microsoft-github-policy-service agree