Add link to conference website(s)

[juliaferraioli]

Even though it's on a different subdomain, we should add a link in the navigation to the conference website. Since the code is not hosted in this repo, we'll need to either:

1. modify the template section-tree-nav, or
2. update the hugo.toml to make use of this feature in the theme

Option 2 is definitely less work and less ongoing maintenance, so that's my suggestion for now.

I'll leave this issue open for about a week before working on it myself, in case anyone would like to make their first contribution to the TLA+ community! Please let me know if you have any questions about the suggested approach, or would like guidance on making your contribution.