doc/mgr: add prompt directives to dashboard.rst

doc/mgr: add prompt directives to dashboard.rst

This commit adds prompt directives (.. prompt:: bash $) to
the commands in dashboard.rst.

There are several ".. include::" directives in the dashboard.rst
file, which means that part of this page is sourced from elsewhere
than the dashboard.rst file. Because I have not yet added prompt
directives to those files, there is an inconsistency in the rendering
of this file. Most of the commands on this page have unselectable
prompts (unselectable prompts are the prompts that don't get added to
the buffer when you copy them to one of the clipboards). But the
commands on this page that come from those ".. include::" directives
do not yet have unselectable prompts.

This file is over 1600 lines long. It was perhaps not optimally wise
of me to have edited all of it in one fell swoop. It took many hours,
and carefully checking it will probably take at least one hour. I
suggest that whoever reviews this should not spend much time on it,
but should instead make a quick pass over the page and make sure that
it looks passable.

The English syntax on this page (and throughout the Dashboard doc-
umentation) will be tightened to remove ambiguity and to improve
readability in the near future, so hold all English-language-related
comments for a future pull request.

Signed-off-by: Zac Dover <zac.dover@gmail.com>