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/JavaScriptEnvironment.md

@@ -65,9 +65,9 @@ Browser
 
 * [console.{log, warn, error, info, trace, table}](https://developer.chrome.com/devtools/docs/console-api)
 * [CommonJS require](https://nodejs.org/docs/latest/api/modules.html)
-* [XMLHttpRequest, fetch](/react-native/docs/network.html#content)
-* [{set, clear}{Timeout, Interval, Immediate}, {request, cancel}AnimationFrame](/react-native/docs/timers.html#content)
-* [navigator.geolocation](/react-native/docs/geolocation.html#content)
+* [XMLHttpRequest, fetch](docs/network.html#content)
+* [{set, clear}{Timeout, Interval, Immediate}, {request, cancel}AnimationFrame](docs/timers.html#content)
+* [navigator.geolocation](docs/geolocation.html#content)
 
 ES6