Update links to doxygen documentation

It was moved from https://doxygen.librepcb.org/master/ to
https://doxygen.librepcb.org/. Branches are now available at
https://doxygen.librepcb.org/_branches/.

CONTRIBUTING.md

@@ -17,7 +17,7 @@
 - Open a new issue for your idea, assuming one does not already exist.
 - Fork the repository on GitHub.
 - Have a look at our [development resources](https://github.com/LibrePCB/LibrePCB/tree/master/dev),
-  especially at the [Doxygen documentation](https://doxygen.librepcb.org/master/).
+  especially at the [Doxygen documentation](https://doxygen.librepcb.org/).
 - When using QtCreator, import and use our [code style guide file](https://github.com/LibrePCB/LibrePCB/blob/master/dev/CodingStyle_QtCreator.xml).
 
 ## Making Changes
@@ -27,7 +27,7 @@
   - To quickly create a topic branch based on master:
     `git checkout -b my_contribution master`
   - Please avoid working directly on the `master` branch.
-- Write code which follows our [code style guides](https://doxygen.librepcb.org/master/df/d24/doc_code_style_guide.html)
+- Write code which follows our [code style guides](https://doxygen.librepcb.org/df/d24/doc_code_style_guide.html)
   and [.editorconfig settings](https://github.com/LibrePCB/LibrePCB/blob/master/.editorconfig).
 - Make commits of logical units.
   - Make sure your commit messages are in the [proper format](http://chris.beams.io/posts/git-commit/):

README.md

@@ -52,7 +52,7 @@ Contributions are welcome! See our [Contributing Guide](CONTRIBUTING.md) for
 details.
 
 For internal details take a look at the
-[automatically generated documentation (doxygen)](https://doxygen.librepcb.org/master/)
+[automatically generated documentation (doxygen)](https://doxygen.librepcb.org/)
 
 ## Development
 

dev/README.md

@@ -1,13 +1,17 @@
 # Development Resources
 
 This directory contains various resources for developers:
+
 - A [demo workspace](https://github.com/LibrePCB/demo-workspace) for testing LibrePCB
 - Some diagrams about the software architecture (class diagrams, library architecture diagrams, ...)
 - Docker containers
-- [Doxygen documentation](https://doxygen.librepcb.org/master/)
+- [Doxygen documentation](https://doxygen.librepcb.org/)
+  ([all branches](https://doxygen.librepcb.org/_branches/))
 - and more...
 
+
 Useful Links:
+
 - [Gerber File Format:](https://www.ucamco.com/en/guest/downloads) [Specifications](https://www.ucamco.com/files/downloads/file/81/the_gerber_file_format_specification.pdf) | [PCB Fabrication Data Guide](https://www.ucamco.com/files/downloads/file/130/pcb%20fabrication%20data%20-%20a%20guide.pdf)
 - Excellon File Format: http://web.archive.org/web/20071030075236/http://www.excellon.com/manuals/program.htm
 - Online 3D PCB Viewer: http://mayhewlabs.com/webGerber/