Move tech design overview from wiki into repo (#12601)
* copy DesignOverview wiki article to devDocs/technicalDesignOverview.md
* break lines at sentence boundaries.
* fix typos / spelling.
* update section on core main loop mentioning the core sleeps when there is no work.
* NVDA Objects section: add a paragraph about relational properties that allow navigating the entire Operating System as a tree.
* textInfos section: note what abilities a TextInfo object has, and also how to fetch them from an NVDA object.
* provide more detailed information about browse mode documents, especially in relation to tree interceptors and virtual buffers, and provide the specific order of how events and scripts are handled by global plugins, app modules, tree interceptors etc.
Co-authored-by: Sean Budd <sean@nvaccess.org>
