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

doc/manual.rst

@@ -1482,7 +1482,8 @@ order. The *names* of the fields also have to be identical.
 
 The assignment operator for tuples copies each component.
 The default assignment operator for objects copies each component. Overloading
-of the assignment operator is described in `type-bound-operations-operator`_.
+of the assignment operator is described `here
+<manual_experimental.html#type-bound-operations>`_.
 
 .. code-block:: nim
 
@@ -2132,7 +2133,7 @@ conversions from ``string`` to ``SQL`` are allowed:
 Now we have compile-time checking against SQL injection attacks.  Since
 ``"".SQL`` is transformed to ``SQL("")`` no new syntax is needed for nice
 looking ``SQL`` string literals. The hypothetical ``SQL`` type actually
-exists in the library as the `TSqlQuery type <db_sqlite.html#TSqlQuery>`_ of
+exists in the library as the `SqlQuery type <db_common.html#SqlQuery>`_ of
 modules like `db_sqlite <db_sqlite.html>`_.
 
 
@@ -2643,7 +2644,7 @@ tuple[x: A, y: B, ...]          (default(A), default(B), ...)
                                 (analogous for objects)
 array[0..., T]                  [default(T), ...]
 range[T]                        default(T); this may be out of the valid range
-T = enum                        cast[T](0); this may be an invalid value
+T = enum                        cast[T]\(0); this may be an invalid value
 ============================    ==============================================
 
 
@@ -3484,8 +3485,8 @@ more argument in this case:
   assert x == y
 
 The command invocation syntax also can't have complex expressions as arguments.
-For example: (`anonymous procs`_), ``if``, ``case`` or ``try``.
-Function calls with no arguments still needs () to
+For example: (`anonymous procs <#procedures-anonymous-procs>`_), ``if``,
+``case`` or ``try``. Function calls with no arguments still needs () to
 distinguish between a call and the function itself as a first class value.
 
 
@@ -3505,8 +3506,8 @@ Creating closures in loops
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Since closures capture local variables by reference it is often not wanted
-behavior inside loop bodies. See `closureScope <system.html#closureScope>`_
-for details on how to change this behavior.
+behavior inside loop bodies. See `closureScope
+<system.html#closureScope.t,untyped>`_ for details on how to change this behavior.
 
 Anonymous Procs
 ---------------
@@ -5914,9 +5915,9 @@ or ``ref T`` or ``ptr T`` this means no locations are modified. It is a static
 error to mark a proc/iterator to have no side effect if the compiler cannot
 verify this.
 
-As a special semantic rule, the built-in `debugEcho <system.html#debugEcho>`_
-pretends to be free of side effects, so that it can be used for debugging
-routines marked as ``noSideEffect``.
+As a special semantic rule, the built-in `debugEcho
+<system.html#debugEcho,varargs[typed,]>`_ pretends to be free of side effects,
+so that it can be used for debugging routines marked as ``noSideEffect``.
 
 ``func`` is syntactic sugar for a proc with no side effects:
 
@@ -6187,7 +6188,8 @@ factor.
 immediate pragma
 ----------------
 
-The immediate pragma is obsolete. See `Typed vs untyped parameters`_.
+The immediate pragma is obsolete. See `Typed vs untyped parameters
+<#templates-typed-vs-untyped-parameters>`_.
 
 
 compilation option pragmas
@@ -7051,7 +7053,7 @@ is not set to C, other pragmas are available:
 
  * `importcpp <manual.html#implementation-specific-pragmas-importcpp-pragma>`_
  * `importobjc <manual.html#implementation-specific-pragmas-importobjc-pragma>`_
- * `importjs <manul.html#implementation-specific-pragmas-importjs-pragma>`_
+ * `importjs <manual.html#implementation-specific-pragmas-importjs-pragma>`_
 
 .. code-block:: Nim
   proc p(s: cstring) {.importc: "prefix$1".}