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/utils/V2Modal.tsx

@@ -22,7 +22,7 @@ export const V2Modal = (): React.ReactElement => {
         You should also take a look at{" "}
         <a
           target="_"
-          href="https://github.com/bitburner-official/bitburner-src/blob/stable/src/Documentation/doc/migrations/v2.md"
+          href="https://github.com/bitburner-official/bitburner-src/blob/stable/src/Documentation/doc/en/migrations/v2.md"
         >
           {" "}
           the migration guide
@@ -30,7 +30,7 @@ export const V2Modal = (): React.ReactElement => {
         as well as{" "}
         <a
           target="_"
-          href="https://github.com/bitburner-official/bitburner-src/blob/stable/src/Documentation/doc/changelog.md"
+          href="https://github.com/bitburner-official/bitburner-src/blob/stable/src/Documentation/doc/en/changelog.md"
         >
           the changelog
         </a>