[doc] Improve pins_if block diagram
[doc] Improve pins_if block diagram

Get rid of the surrounding whitespace, scale up the text and make
stuff bolder. Also get rid of a load of transparent boxes, undo the
text-to-path conversion (which makes things impossible to edit or
search) and generally tidy things up.

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

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