doc: fix genrest turbo mode output

PR #19493 cleaned up the turbo-mode process for doc generation but
introduced an error in the generated index.rst that causes use of the
"make htmldocs-fast" (so-called turbo mode) to fail with an error -
the generated list-table directive has no content (other than
the header row).  This PR adds one dummy row.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>