Don't open external links in a new tab

[jukent]

Expected user input to avoid external links opening in a new tab per MyST parser documentation. MyST does not support all MyST parser configuration settings, so this currently doeesn't work (tried adding it to a specific Cookbook just to test).

The closest I've found so far is url-entries documentation:

- url: 'https://google.com'
  title: Google
  open_in_same_tab: true

I can at least add this setting to the navbar links, but adding it to every link throughout the project is tedious. @agoose77 do you know if there will be any support for a global option to change the default behavior?

[jukent]

Not suggesting we do this as it slows down our website response times, but sample code to overwrite the behavior:

site:
  options:
    html_head: |
      <script>
        document.addEventListener('DOMContentLoaded', function() {
          document.querySelectorAll('a[target="_blank"]').forEach(function(link) {
            link.removeAttribute('target');
          });
        });
      </script>

[jukent]

Closing in favor of #25 specifying open_in_new_tab as False may still be possible if we aren't catching foundations.projectpythia.org in the wildcard.