Add link to github repository in the docs

[ShaiAvr]

I find it slightly annoying that there is no direct link in the documentation to the GitHub repository. Since you are using the pydata theme, it is extremely easy to add links to GitHub and any other external resources which are easily available on the top of the site. Look at the docs of pydata itself for example:

Adding such a link to GitHub is as simple as adding:

html_theme_options = {
    "icon_links": [
        {
            "name": "GitHub",
            "url": "https://github.com/qurit/PyTomography",
            "icon": "fa-brands fa-github",
            "type": "fontawesome",
        },
    ],
}

to conf.py. If there are no objections or issues, I can make a PR to implement this change.