doc: Add links for Split and Single HTML pages

[dbaio]

Summary

[ Split HTML / Single HTML ]

We have this option only for the biggest books. They have this option of
single or split pages, but it's shown for users just the split pages
without the link to access the single HTML.

This is an option we had before.

Test Plan

Tests can be seen here:

• porters-handbook
• fdp-primer

[carlavilla]

Thanks. Go ahead