[doc] Fix rendering of special characters in testplan table
[doc] Fix rendering of special characters in testplan table

The actual change here is quite small (pass 'unsafehtml' to tabulate
when generating HTML output), but the patch is a bit bigger because it
includes comments that nail down exactly what format the text is in at
each stage (it took me a while to figure out!)

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan