Improve Supported Platforms table

[fohrloop]

The current Supported Platforms table lists

• Platform (Windows, macOS, GNOME, KDE Plasma,...)
• Methods
• Modes

It could be improved by

• Renaming "Platform" into "Runtime environment" to underline that this is where code is executed
• ~Adding "Keepawake target system" or similar column telling which system it is targeting to keep awake~ -- Edit: This info seems repetitive. Probably the "Runtime environment" is enough.
• Adding rows for Methods that are not implemented but are under consideration (and have a GitHub Issue). This would make the roadmap / future of wakepy more visible and could attract potential contributors. -- Edit: Decided to create separate table.
• ~Adding examples of distros/OSes for different DEs. This would make searchability a bit better.~ - Edit: decided not to. Updating the DE <-> distro mappings would work. Hopefully the wikipedia links are enough.

New table mockup:

| Runtime environment        | Keepawake target system | Methods                         | Modes|
|----------------------------|-------------------------|---------------------------------|------|
| Windows                    | Windows                 | SetThreadExecutionState         |      |
| WSL                        | Windows                 | wakepy/#NN                      |      |
| cygwin                     | Windows                 | wakepy/#NN                      |      |
| macOS                      | macOS                   | caffeinate                      |      |
| Unix w/ Gnome              | Unix                    | org.gnome.SessionManager        |      | 
| Unix w/ KDE Plasma         | Unix                    | org.freedesktop.PowerManagement |      |
| Unix w/ Cinnamon           | Unix                    | wakepy/#NN                      |      |
| Unix w/ Xfce               | Unix                    | wakepy/#NN                      |      |
| Unix w/ Mate               | Unix                    | wakepy/#NN                      |      |
| Unix w/ LXQt               | Unix                    | wakepy/#NN                      |      |
| Android                    | Android                 | wakepy/#NN                      |      |
| Hosted Jupyter NB          | Any                     | wakepy/#NN                      |      |
| Browser                    | Any                     | wakepy/#NN                      |      |

(Unix would mean Unix-like)