docs: the Windows port, and what is different there

An install subsection alongside macOS and Linux, and a page under
Inside for the rest: how to build it, what changes about running it
(%ComSpec% panes, sessions in %LOCALAPPDATA%), and the reasoning the
code can only gesture at in comments.

The three traps are written down in full, because each one presents as
something other than what it is and cost real time: a pane with a title
and no body (the child inherited slosh's stdout rather than the
pseudoconsole), a pane that stays alive after `exit` (a ConPTY's output
pipe belongs to the console, not the child, so the read never ends), and
an editor refusing a path that plainly exists (cmd.exe and
CommandLineToArgvW disagree about what \" means, and separately the CRT
accepts a mixed-separator path where a GUI dialog will not).

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