emacs docs: rstdoc.el: consistent single quote conversions

emacs docs: rstdoc.el: consistent single quote conversions

With text-quoting-style 'grave keeps "'" and "`" quotes unaltered
for further processing done by this code (regardless of locale...).
The tools that read the reStructuredText markup generated can do
their styling instead.

Added temporary conversions of ' and ` to \001 and \002 so that
's and `s outside of `...' and `...` are converted separately
('s restored back to ' and `s converted to \`).

Both `...' and `...` are finally "converted" to `...` (not ``...``).
https://docutils.sourceforge.io/docs/user/rst/quickref.html documents
that as `interpreted text`:

 "The rendering and meaning of interpreted text is domain- or
  application-dependent. It can be used for things like index
  entries or explicit descriptive markup (like program identifiers)."

Which looks pretty much right.