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