Move ios documentation from //docs to //docs/ios.

docs/ios/build_instructions.md

 # Checking out and building Chromium for iOS
 
 There are instructions for other platforms linked from the
-[get the code](get_the_code.md) page.
+[get the code](../get_the_code.md) page.
 
 ## Instructions for Google Employees
 
 Are you a Google employee? See
 [go/building-chrome](https://goto.google.com/building-chrome) instead.
 
 [TOC]
 
 ## System requirements
 
@@ skipping 80 matching lines @@
 
 ```shell
 $ ninja -C out/Debug-iphonesimulator gn_all
 ```
 
 Note: you need to run `setup-gn.py` script every time one of the `BUILD.gn`
 file is updated (either by you or after rebasing). If you forget to run it,
 the list of targets and files in the Xcode solution may be stale.
 
 You can also follow the manual instructions on the
-[Mac page](mac_build_instructions.md), but make sure you set the
+[Mac page](../mac_build_instructions.md), but make sure you set the
 GN arg `target_os="ios"`.
 
 ## Building for device
 
 To be able to build and run Chromium and the tests for devices, you need to
 have an Apple developer account (a free one will work) and the appropriate
 provisioning profiles, then configure the build to use them.
 
 ### Code signing identity
 
@@ skipping 190 matching lines @@
 
 ```shell
 $ xcodebuild -license
 ```
 
 Only accepting for all users of the machine requires root:
 
 ```shell
 $ sudo xcodebuild -license
 ```