fix several broken links

doc/manual/type_rel.txt

@@ -313,7 +313,7 @@ matches better than just ``T`` then.
 Automatic dereferencing
 -----------------------
 
-If the `experimental mode <experimental pragma>`_ is active and no other match
+If the `experimental mode <#pragmas-experimental-pragma>`_ is active and no other match
 is found, the first argument ``a`` is dereferenced automatically if it's a
 pointer type and overloading resolution is tried with ``a[]`` instead.
 

lib/system/nimscript.nim

@@ -234,7 +234,7 @@ proc cd*(dir: string) {.raises: [OSError].} =
   ##
   ## The change is permanent for the rest of the execution, since this is just
   ## a shortcut for `os.setCurrentDir()
-  ## <http://nim-lang.org/os.html#setCurrentDir,string>`_ . Use the `withDir()
+  ## <http://nim-lang.org/docs/os.html#setCurrentDir,string>`_ . Use the `withDir()
   ## <#withDir>`_ template if you want to perform a temporary change only.
   setCurrentDir(dir)
   checkOsError()

readme.md

@@ -49,7 +49,7 @@ The above steps can be performed on Windows in a similar fashion, the
 instead of ``build.sh``.
 
 The ``koch`` tool is the Nim build tool, more ``koch`` related options are
-documented in [doc/koch.txt](doc/koch.txt).
+documented in [doc/koch.rst](doc/koch.rst).
 
 ## Nimble
 [Nimble](https://github.com/nim-lang/nimble) is Nim's package manager. For the