Add link to new procedure for abandonded translation groups from "Dealing with unsupported (abandoned) projects"

On Dealing with unsupported (abandoned) projects, please add a link to the new handbook page Dealing with abandoned translation groups.

The procedure was discussed at #1858786: We need a procedure for appointing new admins to abandoned translation groups. Since translation groups are managed a bit differently from other projects (several admins/managers, and several levels of contributor permissions), we couldn't simply use the same procecure as for other projects. For an example of a translation group which needs this procedure, see #1793084: Nepali Translation is not actively maintained.