doc: remove changelog

Change logs are now hosted on Github releases page.

Closes #4142

Author: igrr

doc/changes.rst

@@ -20,4 +20,3 @@ Welcome to ESP8266 Arduino Core's documentation!
    Debugging <Troubleshooting/debugging>
    Stack Dump <Troubleshooting/stack_dump>
    Using with Eclipse <eclipse/eclipse>
-   Changelog <changes>