Ensure documentation has consistent relative links

Summary:
Explain the **motivation** for making this change. What existing problem does the pull request solve?

I had tried fixing a broken link in a previous commit (#11453). My commit was merged, but it did not resolve the underlying problem. I have looked into how links should be formed for the docs and have fixed the original problem as well as updated all other links to be consistent.

Previous link formats:

- /docs/sample.html <-- broken link
- sample.html <-- broken link
- https://facebook.github.io/react-native/docs/sample.html <-- works
- /react-native/docs/sample.html <-- works
- docs/sample.html <-- works (permalink format)

This PR updates all links to the permalink format.

**Test plan (required)**
I ran the website locally and manually tested half of the links in each category. They all worked.
```
$ cd website
$ npm install && npm start
```
Closes https://github.com/facebook/react-native/pull/12064

Differential Revision: D4489153

Pulled By: mkonicek

fbshipit-source-id: bf0231d941ba147317595c3b3466dc579a887169

CameraRoll.js

@@ -111,7 +111,7 @@ var getPhotosReturnChecker = createStrictShapeTypeChecker({
 /**
  * `CameraRoll` provides access to the local camera roll / gallery.
  * Before using this you must link the `RCTCameraRoll` library.
- * You can refer to [Linking](https://facebook.github.io/react-native/docs/linking-libraries-ios.html) for help.
+ * You can refer to [Linking](docs/linking-libraries-ios.html) for help.
  *
  * ### Permissions
  * The user's permission is required in order to access the Camera Roll on devices running iOS 10 or later.