docs: add HTTPS clone option alongside SSH in README

[NethmikaKekuu]

Summary

Adds the HTTPS clone URL as an alternative to the existing SSH clone command in the installation instructions.

Motivation

The README's "Installation" section only shows the SSH clone command (git@github.com:microsoft/markitdown.git). This works fine for contributors who already have an SSH key configured with GitHub, but new users without SSH keys set up hit a confusing error:

git@github.com: Permission denied (publickey).
fatal: Could not read from remote repository.

Since this is a public repo, HTTPS cloning works with no setup at all. Adding it alongside SSH makes the "Installation" section easier to follow for first-time users, while keeping the SSH option for those who prefer it.

Change

# HTTPS 
git clone https://github.com/microsoft/markitdown.git
# SSH 
git clone git@github.com:microsoft/markitdown.git
cd markitdown
pip install -e 'packages/markitdown[all]'

Checklist

• Documentation-only change, no code affected
• Verified both clone commands work as expected