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/ui/React/Documentation.tsx

@@ -24,7 +24,7 @@ const defaultPage = asFilePath("index.md");
 export const defaultNsApiPage = asFilePath("nsDoc/bitburner.ns.md");
 /**
  * If we move or rename "bitburner.ns.md", we must update this constant, "defaultNsApiPage", "openDocExternally", and
- * the URL in src\Documentation\doc\index.md.
+ * the URL in src/Documentation/doc/en/index.md.
  */
 export const externalUrlOfNsApiPage =
   "https://github.com/bitburner-official/bitburner-src/blob/stable/markdown/bitburner.ns.md";