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

docs/LayoutWithFlexbox.md

@@ -104,4 +104,4 @@ AppRegistry.registerComponent('AwesomeProject', () => AlignItemsBasics);
 
 We've covered the basics, but there are many other styles you may need for layouts. The full list of props that control layout is documented [here](./docs/layout-props.html).
 
-We're getting close to being able to build a real application. One thing we are still missing is a way to take user input, so let's move on to [learn how to handle text input with the TextInput component](/react-native/docs/handling-text-input.html).
+We're getting close to being able to build a real application. One thing we are still missing is a way to take user input, so let's move on to [learn how to handle text input with the TextInput component](docs/handling-text-input.html).