Move man page creation to markdown syntax. This allows for easier
editing than groff syntax.

Old: https://spdx.org/licenses/GPL-3.0.html
New: https://spdx.org/licenses/GPL-3.0-only.html

- Add missing releases
- Add recommended input devices
- Improve summary based on recommendations from [1]

[1]: https://blogs.gnome.org/tbernard/2021/09/07/ready-for-software-41/

Signal to the desktop that gajim does not support having an additional
main window opended.

https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s06.html

- Feature is broken since mid 2020
- No demand since then to fix it