docs: fix links, remove some weird ones (#2062)

Fixes docs links and removes some weird ones.

Closes #2061.

website/content/blog/netlify-cms-2-1-0-adds-external-media-support-with-uploadcare.md

@@ -31,8 +31,6 @@ Uploadcare came up on our radar when they built a Netlify CMS integration for th
 
 We're super excited to honor Uploadcare's love for Netlify CMS by launching external media handling with their service as the first integration!
 
-You can play with the integration using a test account [right now](https://5b85d3054ed62f29fd759737--cms-demo.netlify.com/#/collections/posts/new), or keep reading to get Uploadcare working on your Netlify CMS site!
-
 ## Get started today!
 
 Here's how to start externalizing your media with Netlify CMS and Uploadcare:

website/content/docs/customization.md

@@ -9,8 +9,6 @@ The NetlifyCMS exposes a `window.CMS` global object that you can use to register
 * **registerPreviewStyle:** Register a custom stylesheet to use on the preview pane.
 * **registerPreviewTemplate:** Registers a template for a collection.
 
-Explore the [NetlifyCMS GitHub example](https://github.com/netlify/netlify-cms/blob/9ced3f16c8bcc3d1a36773b126842d023c589eaf/example/index.html#L90-L91), a working example you can review on GitHub.
-
 ### React Components inline interaction
 
 NetlifyCMS is a collection of React components and exposes two constructs globally to allow you to create components inline: ‘createClass’ and ‘h’ (alias for React.createElement).

website/content/docs/intro.md

@@ -35,4 +35,4 @@ If you hook up Netlify CMS to your website, you're basically adding a tool for c
 - [Start with a template](../start-with-a-template/) to make a Netlify CMS-enabled site of your own.
 - Configure your existing site by following a [tutorial](../add-to-your-site/) or checking [configuration options](../configuration-options).
 - Ask questions and share ideas in the Netlify CMS community on [Gitter](https://gitter.im/netlify/netlifycms).
-- Get involved in new developments and become a [contributor](https://docs-starters--netlify-cms-www.netlify.com/docs/contributor-guide/).
+- Get involved in new developments and become a [contributor](../contributor-guide/).

website/src/components/header.js

@@ -60,16 +60,16 @@ class Header extends Component {
                   {isDocs && <DocSearch />}
                 </div>
                 <div className="nav-container">
-                  <Link className="nav-link docs-link" to="/docs/intro">
+                  <Link className="nav-link docs-link" to="/docs/intro/">
                     Docs
                   </Link>
-                  <Link className="nav-link contributing-link" to="/docs/contributor-guide">
+                  <Link className="nav-link contributing-link" to="/docs/contributor-guide/">
                     Contributing
                   </Link>
-                  <Link className="nav-link" to="/community">
+                  <Link className="nav-link" to="/community/">
                     Community
                   </Link>
-                  <Link className="nav-link" to="/blog">
+                  <Link className="nav-link" to="/blog/">
                     Blog
                   </Link>
                   <span className="gh-button">