[doc] Minor tweak to md sanitisation code

[rswarbrick]

This code replaces \n with a space when populating table cells. A good
idea! But it ended up leaving a double space if the source hjson had a
blank line.

There was also a bit of a mess if we have a newline followed by several
spaces. This turned into two spaces, not one.

The fix is simple: treat any number of newlines, followed by any
number of spaces as a single space. The handwritten code here is just
a 1-line change to md_helpers.py. The rest of the commit is from
regenerating the auto-generated stuff that it builds.

Note that this was annoying to create because we check in the
auto-generated registers.md for pwrmgr in a template file (presumably
manually) and then use that auto-generated file to create the
registers.md in ip_autogen. We should probably stop doing that silly
thing at some point, but my local hack was to apply the fix to the
markdown files with

util/cmdgen.py -u '**/*.md'

and then manually copy the changed lines to the template file before
re-running

make -C hw top

This works, but... yuck!

[rswarbrick]

This fixes one of the two things I found for a single IP and hackily semi-fixed in #20734. This attempt should be a bit more sensible.

EDIT: Actually, it fixes both! They both turned out to be the same problem, now sorted.

[rswarbrick]

Oof, this was painful to get right. The problem is that we automatically generated some Markdown that we then renamed to hw/ip_templates/pwrmgr/doc/registers.md.tpl and then checked in (see commit 8034e00). This doesn't get automatically updated with the automatic cmdgen script, and running that script manually on the tpl file also doesn't work.

Grr, what a bodge! Anyway, fixing things manually seems to get things sorted again for now.

[matutem]

Seems good. I trust you build the docs and did some perusing of the affected files.