doc: add deprecations page to docs toc

PR-URL: https://github.com/nodejs/node/pull/12268
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/_toc.md

@@ -14,6 +14,7 @@
 * [Console](console.html)
 * [Crypto](crypto.html)
 * [Debugger](debugger.html)
+* [Deprecated APIs](deprecations.html)
 * [DNS](dns.html)
 * [Domain](domain.html)
 * [Errors](errors.html)