Fix many broken links and prefer relative links within docs (#12463)

* Fix many broken links

Note that contrary to what docgen.rst currently says, the ids have
to match exactly or else most web browsers will not jump to the
intended symbol.

* Prefer relative links for Nim documentation

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.

* koch.rst: remove link to nonexistent section

* manual.rst: remove unintended link

cast[T](0) is interpreted as a link to id 0 with text T, so escape
the opening parentheses to display the intended output.

* asyncstreams: replace unintended link with emphasis

* Fix word wrapping

lib/pure/math.nim

@@ -35,7 +35,7 @@
 ##   # nan   (use `complex` module)
 ##
 ## This module is available for the `JavaScript target
-## <backends.html#the-javascript-target>`_.
+## <backends.html#backends-the-javascript-target>`_.
 ##
 ## **See also:**
 ## * `complex module<complex.html>`_ for complex numbers and their