Fix links that point to old documentation (CONTRIBUTING and README)

Signed-off-by: Fabio Sfuncia <fsfuncia@unime.it>

CONTRIBUTING.md

@@ -55,7 +55,7 @@ The core team will review your pull request and will either merge it, request ch
 
 To facilitate the contribution, we have drastically reduced the amount of commands necessary to install the entire development environment.
 
-First of all, you need to check if you're using the [required versions of Node.js and npm](https://strapi.io/documentation/3.0.0-beta.x/getting-started/install-requirements.html)
+First of all, you need to check if you're using the [required versions of Node.js and npm](https://strapi.io/documentation/v3.x/installation/cli.html#step-1-make-sure-requirements-are-met)
 
 Then, please follow the instructions below:
 

README.md

@@ -60,7 +60,7 @@ Strapi is a free and open source headless CMS delivering your content anywhere y
 
 ## Getting Started
 
-<a href="https://strapi.io/documentation/3.0.0-beta.x/getting-started/quick-start.html" target="_blank">Read the Getting Started tutorial</a> or follow the steps below:
+<a href="https://strapi.io/documentation/v3.x/getting-started/quick-start.html" target="_blank">Read the Getting Started tutorial</a> or follow the steps below:
 
 ### ⏳ Installation
 
@@ -86,7 +86,7 @@ Enjoy 🎉
 
 ### 🖐 Requirements
 
-Complete installation requirements can be found in the documentation under <a href="https://strapi.io/documentation/3.0.0-beta.x/getting-started/install-requirements.html">Installation Requirements</a>.
+Complete installation requirements can be found in the documentation under <a href="https://strapi.io/documentation/v3.x/installation/cli.html#step-1-make-sure-requirements-are-met">Installation Requirements</a>.
 
 **Supported operating systems**:
 
@@ -145,7 +145,7 @@ For general help using Strapi, please refer to [the official Strapi documentatio
 
 ## Migration
 
-Follow our [migration guides](https://strapi.io/documentation/3.0.0-beta.x/migration-guide/) on the documentation to keep your projects up-to-date.
+Follow our [migration guides](https://strapi.io/documentation/v3.x/migration-guide/#migrations-guides) on the documentation to keep your projects up-to-date.
 
 ## Roadmap