This PR adds:
- link to the Heredoc refdoc (always handy for not straightforward
concepts)
- chapter about Herestring
- chapter about Process Substitution (I know it is already tackled in
the _Redirection_ chapter but I believe it deserves its own one...
Congrats for this cheatsheet, by far my most beloved one ever !!
---------
Co-authored-by: Rico Sta. Cruz <rstacruz@users.noreply.github.com>
