docs/markdown: Switch to using pandoc, and fix underscore escaping

docs/markdown: Switch to using pandoc, and fix underscore escaping

c/s a3a99df44 "docs/cmdline: Rewrite the cpuid_mask_* section" completely
forgot about how markdown gets rendered to HTML (as opposed to PDF), because
we use different translators depending on the destination format.

markdown and pandoc are very similar markup languages, but a couple of details
about pandoc cause it to have far more user-friendly inline markup.

Switch all markdown documents to be pandoc (so we are using a single
translator, and therefore a single flavour of markdown), which fixes the
rendered docs on xenbits.xen.org/docs.

While changing the format, fix the remainder of the escaped underscores in the
same mannor as the previous patch.  The two problem cases here are __LINE__
and __FILE__ where the first underscore still needs escaping.

In addition, dmop.markdown and dom0less.markdown didn't used to get processed,
as only .markdown files in the misc/ directory got considered.
dom0less.pandoc gets picked up automatically now, due to being in the
features/ directory, but designs/ needs adding to the pandoc directory list
for dmop.pandoc to get processed.

While edting in appropriate areas, take the opportunity to fix some markup to
the surrounding style, and drop trailing whitespace.

No change in content - only formatting.  This results in the text being easier
to read and grep.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Ian Jackson <ian.jackson@eu.citrix.com>