feature: pages index.

By having an index for the pages, it'd allow clients
to build autocomplete's and show an index of pages
making only one request (think `tldr list`). Subsequent
caching of this file would prevent further requests.

Included is a makefile to easily rebuild the index.
It could be tailored in case the pages structure changed.

In a particular case, the web client would benefit greatly
from an index. It'd know exactly what folder a command is in
and thus the number of requests would drop from 4 to 1.

Makefile

@@ -0,0 +1,3 @@
+index:
+	ls -d1 ./pages/**/* | grep .md | awk -F"." '{print $$2}' | awk -F"/" '{print $$4,$$3}' | sort > pages/index.md
+	@echo "Index rebuilt."

pages/index.md

@@ -0,0 +1,189 @@
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