docs: Add Doxygen and project-related documentation

core/doc/contributing.md

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+\page contributing Contributing
+\brief All resources for (potential) contributors
+
+This page should contain all resources for people who'd like to contribute to the project. 
+If that happens to be you, thank you for considering to contribute to this project! 
+This page will describe how the process goes, what code style this project uses and more useful information.
+
+# Development process
+Development happens at [GitHub](https://github.com/heartfin/harbour-sailfin). 
+Currently, both the library Jellyfin-Qt and the [Sailfish OS](https://sailfishos.org) app named Sailfin are developed in the same repository.
+
+## Create or find an issue to work on
+If you want to contribute code, please check if an issue exists. 
+Otherwise, create an issue describing what feature you'd like to develop before creating a merge request.
+
+## Start working on your code
+Clone the git repo and create a new branch. 
+If you have commit access to the repository, please create a branch in the form of `<issue-number>-<issue-title>`. 
+For example, `36-control-remote-jellyfin-sessions`.
+
+Now you can start writing your code and create commits. 
+Please create focussed commits, that is, do not commit anything not related to the issue you are working on.
+Also state what you're changing in the commit message and add an entry in the file `rpm/harbour-sailfin.changes`.
+
+If this is your first time contributing to the project, don't be shy and feel free to add your name to the contributors list in `sailfish/qml/pages/AboutPage.qml`!
+
+## Submit a merge request
+Now that you have finalised your commit, you can create a merge request on the repository.
+Make sure that you have followed the contribution guidelines on this page.
+Someone will review it and if all is good, they will merge it in.
+
+Congratulations on your contribution!
+
+# Code style guidelines
+The code should follow the [Sailfish OS Code Conventions](https://docs.sailfishos.org/Develop/Apps/Coding_Conventions/) with the following exceptions:
+
+## Exceptions for C++
+- Put curly braces on the same line as the declaration, example:
+  ```cpp
+  int foo() {
+      // implementation
+  }
+  ```
+  instead of
+  ```cpp
+  int foo()
+  {
+     // implementation
+  }
+  ```
+  Rationale: I hate pressing enter and am used to the former. Moreover, it is already all over the place. Bad rationale, I know.
+
+- Close namespace brackets with a comment in the form `// NS Jellyfin`, example:
+  ```cpp
+  namespace Jellyfin {
+  namespace Nested {
+      // Content
+  } // NS Nested
+  } // NS Jellyfin
+  ```
+  Rationale: Easy to know what namespace the bracket closes, since namespaces tend to span over large portions of a file.
+
+- Use include guards in the form of `NAMESPACE_CLASSNAME_H`. Example:
+  ```cpp
+  #ifndef JELLYFIN_MODEL_USER_H
+  #define JELLYFIN_MODEL_USER_H
+  namespace Jellyfin {
+  namespace Model {
+
+  class User {
+      // Methods etc
+  };
+
+  } // NS Model
+  } // NS Jellyfin
+  ```
+