Fix getting started links

Author: josevalim

lib/elixir/lib/gen_server.ex

@@ -475,7 +475,7 @@ defmodule GenServer do
   guide provides a tutorial-like introduction. The documentation and links
   in Erlang can also provide extra insight.
 
-    * [GenServer - Elixir's Getting Started Guide](https://elixir-lang.org/getting-started/mix-otp/genserver.html)
+    * [GenServer - Elixir's Getting Started Guide](genservers.md)
     * [`:gen_server` module documentation](`:gen_server`)
     * [gen_server Behaviour - OTP Design Principles](https://www.erlang.org/doc/design_principles/gen_server_concepts.html)
     * [Clients and Servers - Learn You Some Erlang for Great Good!](http://learnyousomeerlang.com/clients-and-servers)

lib/elixir/lib/process.ex

@@ -504,7 +504,7 @@ defmodule Process do
   If the process is already dead when calling `Process.monitor/1`, a
   `:DOWN` message is delivered immediately.
 
-  See ["The need for monitoring"](https://elixir-lang.org/getting-started/mix-otp/genserver.html#the-need-for-monitoring)
+  See ["The need for monitoring"](genservers.md#the-need-for-monitoring)
   for an example. See `:erlang.monitor/2` for more information.
 
   Inlined by the compiler.

lib/elixir/pages/references/syntax-reference.md

@@ -395,7 +395,7 @@ end
 
 All of the constructs above are part of Elixir's syntax and have their own representation as part of the Elixir AST. This section will discuss the remaining constructs that are alternative representations of the constructs above. In other words, the constructs below can be represented in more than one way in your Elixir code and retain AST equivalence. We call this "Optional Syntax".
 
-For a lightweight introduction to Elixir's Optional Syntax, [see this document](https://elixir-lang.org/getting-started/optional-syntax.html). Below we continue with a more complete reference.
+For a lightweight introduction to Elixir's Optional Syntax, [see this document](optional-syntax.md). Below we continue with a more complete reference.
 
 ### Integers in other bases and Unicode code points