[WEBSITE] Don't show menu categories when there is a single one

Summary:
The man pages have a Bitcoin ABC menu category, but no other. This
diff the categories display when there is a single one.

Test Plan: Build the website and navigate through the documentation.

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7334