* Update Documentation about GC, add ARC based on code observations and https://forum.nim-lang.org/t/5734#35562
* Rephrase 'Garbage Collector' on documentation, see #13331
* Peer Review Feedbacks
* When the body of a documentation section is all for garbage collector details then add 'for garbage collectors' to the title to identify they may not apply for ARC
* Change 'GC' for 'garbage collector'.
* Move 'Memory Management Strategies' at the top, without changes
* Move the info for RefC GC from 'Introduction' to 'RefC' section without changes, make 'Introduction' about Nims 'Multi-paradigm Memory Management Strategies'.
This is more friendly to those browsing the documentation without
a network connection. The nim-doc package in Debian allows this,
for example.
Also, the domain name being used was not consistent. It could have
been either nim-lang.org or nim-lang.github.io, and those reading
the stable docs could have found themselves suddenly reading the
devel docs instead.
