Added markdownlint to Github CI

- Markdown will now be validated as part of Github CI
- Applied auto-fix to most common issues (mostly whitespace related)
- Fixed manually some of more complex cases

Some valid markdownlint rules are currently disabled, to avoid failing
CI, can be enabled in future

docs/developers/Development_with_Qt_Creator.md

@@ -6,7 +6,7 @@ Qt Creator is the recommended IDE for VCMI development on Linux distributions, b
 - Almost no manual configuration when used with CMake. Project configuration is read from CMake text files,
 - Easy to setup and use with multiple different compiler toolchains: GCC, Visual Studio, Clang
 
-You can install Qt Creator from repository, but better to stick to latest version from Qt website: https://www.qt.io/download-qt-installer-oss
+You can install Qt Creator from repository, but better to stick to latest version from Qt website: <https://www.qt.io/download-qt-installer-oss>
 
 ## Configuration
 
@@ -21,4 +21,4 @@ The build dir should be set to something like /trunk/build for the debug build a
 There is a problem with QtCreator when debugging both vcmiclient and vcmiserver. If you debug the vcmiclient, start a game, attach the vcmiserver process to the gdb debugger(Debug \> Start Debugging \> Attach to Running External Application...) then breakpoints which are set for vcmiserver will be ignored. This looks like a bug, in any case it's not intuitively. Two workarounds are available luckily:
 
 1. Run vcmiclient (no debug mode), then attach server process to the debugger
-2. Open two instances of QtCreator and debug vcmiserver and vcmiclient separately(it works!)
+2. Open two instances of QtCreator and debug vcmiserver and vcmiclient separately(it works!)