DOC: Move all docs into en/ subdirectory (#1505)

* DOC: Move all docs into en/ subdirectory

PR #1502 is working on adding a Chinese translation to the docs. In
general, I encouraged this (in #1452) as a path towards getting useful
translated content in the game without requiring a massive
refactor/rearchitecting of everything.

To support this, this takes the first step of moving our docs into an
en/ subdirectory, so that other languages can live alongside. No effort
is made at this time to support or select between alternate languages;
this is a pure-rename refactor.

src/Documentation/doc/en/advanced/intelligence.md

@@ -0,0 +1,17 @@
+# Intelligence
+
+Intelligence is a stat that is unlocked by having [Source-File 5](sourcefiles.md) (i.e. Destroying [BitNode-5](bitnodes.md)).
+
+Intelligence is unique because it is permanent and persistent.
+It never gets reset back to 1.
+However, gaining Intelligence experience is extremely slow.
+It is a stat that gradually builds up as you continue to play the game.
+
+Intelligence will boost your production for many actions in the game, including:
+
+- [Hacking](../basic/hacking.md)
+- [Infiltration](../basic/infiltration.md)
+- [Crime](../basic/crimes.md) success rate
+- [Bladeburner](bladeburners.md) actions
+- [Reputation](../basic/reputation.md) gain for [Companies](../basic/companies.md) & [Factions](../basic/factions.md)
+- [Augmentation](../basic/augmentations.md) [Grafting](grafting.md) speed