Zsh and Prezto files don't always reside in `~/`. Instead, they have
a more spec compliant location (`${ZDOTDIR:-$HOME}/`). Make them
unambiguous in the documentation. Likewise, for `$GNUPGHOME`.
Further, add instruction for optionally setting up Prezto in
`$XDG_CONFIG_HOME`.
While at this, also add clarification on relative ordering of some of
the modules, apply more formatting tweaks and doc improvements.
General purpose reformating and rewording in (a few places). This should
also make markdown linting happier.
Prefer back-ticks wherever useful and use `console` for code blocks to
be executed on terminal.
Also, update references links wherever possible (including update from
'http://' to 'https://').
