Bash: add Herestring & Process substitution chapters (#2184)

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 <[email protected]>

Author: pismy

bash.md

@@ -791,6 +791,30 @@ hello world
 END
 ```
 
+Heredoc allows a section of your source code to be treated as a file. See [Bash Reference Manual](https://www.gnu.org/software/bash/manual/html_node/Redirections.html#Here-Documents).
+
+### Herestring
+
+```sh
+tr '[:lower:]' '[:upper:]' <<< "Will be uppercased, even $variable"
+```
+
+Herestring allows a string to be treated as a standard input (stdin). See [Bash Reference Manual](https://www.gnu.org/software/bash/manual/html_node/Redirections.html#Here-Strings).
+
+### Process substitution
+
+```sh 
+# loop on myfunc output lines
+while read -r line; do
+  echo "$line"
+done < <(myfunc)
+
+# compare content of two folders
+diff <(ls "$dir1") <(ls "$dir2")
+```
+
+Process substitution allows the input (or output) of a command to be treated as a file. See [Bash Reference Manual](https://www.gnu.org/software/bash/manual/html_node/Process-Substitution.html).
+
 ### Reading input
 
 ```bash