This MR moves content from the following places to Develop:

* https://community.kde.org/Get_Involved/development
* https://community.kde.org/Get_Involved/development/Set_up_a_development_environment
* https://community.kde.org/Get_Involved/development/Build_software_with_kdesrc-build
* https://community.kde.org/Get_Involved/development/Choose_what_to_work_on (a draft I attempted at some point)
* https://community.kde.org/Get_Involved/development/Install_the_dependencies

The following is out of scope for this MR:

* https://community.kde.org/Infrastructure/GitLab (this can be done in a later MR)
* https://community.kde.org/Get_Involved/development/More (this can be done in a later MR)
* teams/web/wiki-sites#3 (while the structure is there, the content itself relies moving stuff from development/More and can be done in a later MR)

This is in a state where it can finally be viewed: https://kdocs.rabbitictranslator.com/kdesrc-build/public/docs/getting-started/building/