Add button to GitHub Repo

I personally dislike it a lot when I come to the documentation of a source code, but there is no easy/quick link to the repo, which means I have to google to find the repo ;-)

Adding the `"github_url" to the "html_theme_options" should add a GitHub-link-button on the top-right corner, next to the switch for light/dark theme.

docs/source/conf.py

@@ -124,6 +124,7 @@
 # Theme config
 html_theme = "pydata_sphinx_theme"
 html_theme_options = {
+    "github_url": "https://github.com/PyLops/pylops",
     # "logo_only": True,
     # "display_version": True,
     "logo": {