Update dead/broken README links (#13931)

Signed-off-by: Danielle De Leo <danielle@fastmail.net>

README.md

@@ -68,10 +68,10 @@ See [our roadmap](https://github.com/eclipse-theia/theia/wiki/Eclipse-Theia-Road
 Here you can find guides and examples for common scenarios to adopt Theia:
 
 - [Get an overview of how to get started](https://theia-ide.org/#gettingstarted) on the Theia website
-- [Develop a Theia application - your own IDE/Tool](https://www.theia-ide.org/doc/Composing_Applications.html)
+- [Develop a Theia application - your own IDE/Tool](https://theia-ide.org/docs/composing_applications/)
 - [Learn about Theia's extension mechanisms](https://theia-ide.org/docs/extensions/)
 - [Develop a VS Code like extension](https://theia-ide.org/docs/authoring_vscode_extensions/)
-- [Develop a Theia extension](http://www.theia-ide.org/doc/Authoring_Extensions.html)
+- [Develop a Theia extension](https://theia-ide.org/docs/authoring_extensions/)
 - [Test a VS Code extension in Theia](https://github.com/eclipse-theia/theia/wiki/Testing-VS-Code-extensions)
 - [Package a desktop Theia application with Electron](https://theia-ide.org/docs/blueprint_documentation/)
 
@@ -84,7 +84,7 @@ Read below to learn how to take part in improving Theia:
 - Find an issue to work on and submit a pull request
   - First time contributing to open source? Pick a [good first issue](https://github.com/eclipse-theia/theia/labels/good%20first%20issue) to get you familiar with GitHub contributing process.
   - First time contributing to Theia? Pick a [beginner friendly issue](https://github.com/eclipse-theia/theia/labels/beginners) to get you familiar with codebase and our contributing process.
-  - Want to become a Committer? Solve an issue showing that you understand Theia objectives and architecture. [Here](https://github.com/eclipse-theia/theia/labels/help%20wanted) is a good list to start. Further, have a look at our [roadmap](https://github.com/eclipse-theia/theia/wiki/Roadmap) to align your contributions with the current project goals.
+  - Want to become a Committer? Solve an issue showing that you understand Theia objectives and architecture. [Here](https://github.com/eclipse-theia/theia/labels/help%20wanted) is a good list to start. Further, have a look at our [roadmap](https://github.com/eclipse-theia/theia/wiki/Eclipse-Theia-Roadmap) to align your contributions with the current project goals.
 - Could not find an issue? Look for bugs, typos, and missing features.
 
 ## Feedback
@@ -115,7 +115,7 @@ Read below how to engage with Theia community:
 ## License
 
 - [Eclipse Public License 2.0](LICENSE-EPL)
-- [一 (Secondary) GNU General Public License, version 2 with the GNU Classpath Exception](LICENSE-GPL-W-CLASSPATH-EXCEPTION)
+- [一 (Secondary) GNU General Public License, version 2 with the GNU Classpath Exception](LICENSE-GPL-2.0-ONLY-CLASSPATH-EXCEPTION)
 
 ## Trademark