Fixup docs index font sizes
Summary:
This patch fixes the font sizes so the doc index table displays better on smaller
screen sizes, as well as making room for more columns (see D7457).
Test Plan:
make serve
Navigate to http://127.0.0.1:4000/doc/
Verify it renders nicely on many screen sizes.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D7459