Moved some documentation files to docs directory

CONTRIBUTING.md

@@ -6,7 +6,7 @@ So you want to contribute to Colobot: Gold Edition? That's awesome! Before you s
 
 Before you start, read more about technical details of the project. They can be found in:
 
-* [Developer README](README-dev.md)
+* [Developer README](docs/README-dev.md)
 * [Doxygen documentation (doc)](https://github.com/colobot/colobot/actions)
 * [Working with translations](https://github.com/colobot/colobot/wiki/Working-with-translations)
 * [Working with models](https://github.com/colobot/colobot/wiki/Working-with-Colobot:-Gold-Edition-models)

INSTALL.md

@@ -31,7 +31,7 @@ You can also compile directly on Windows with MSYS2/MinGW-w64 or Visual Studio s
 
 #### Compiling with MSYS2/MinGW-w64
 
-See the [INSTALL-MSYS2.md](INSTALL-MSYS2.md) file for details.
+See the [INSTALL-MSYS2.md](docs/INSTALL-MSYS2.md) file for details.
 
 #### Compiling with MSVC (Visual Studio)
 
@@ -115,7 +115,7 @@ So if you provided prefix "/some/prefix", you can run:
 
 ### Compiling on MacOS X
 
-As of 0.1.2-alpha, we have added MacOS X support. See [INSTALL-MacOSX.md](INSTALL-MacOSX.md)
+As of 0.1.2-alpha, we have added MacOS X support. See [INSTALL-MacOSX.md](docs/INSTALL-MacOSX.md)
 file for details.