Document modules for supporting Wayland more clearly

[Game4Move78]

The following information is easy to miss:

• Wayland not yet supported by aw-watcher-window or aw-watcher-afk.
• Wayland (Posh, Sway) is supported by aw-watcher-wayland (window+idle).
• Wayland (Sway, Hyprland, KDE, GNOME, and X11) is supported by awatcher (window+idle).

Wayland users are likely to burn themselves once or twice before realising the default watchers do not support their system. Currently this is hinted with (X11 only) in the "Window Watchers" section of watchers.rst, which users may not think to check, so I added a note on the "Getting Started" and "Installing from Source" pages. Information that aw-watcher-window-wayland doesn't work on KDE or Gnome is also not clear, so I added a list of the supported DE in brackets.

Also I linked to "building from source" as an installation option. At the moment it is only linked to from "Usage", which is less intuitive.

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[!IMPORTANT] Improves documentation clarity on Wayland support and installation options in ActivityWatch.

• Documentation Updates:
  • Added notes in getting-started.rst and installing-from-source.rst about Wayland not being supported by default watchers, with links to Wayland-compatible watchers.
  • Updated watchers.rst to clarify default watcher support (Windows, macOS, Linux X11) and list Wayland-compatible watchers (aw-watcher-window-wayland, 2e3s/awatcher).
• Installation Guidance:
  • Linked "building from source" as an installation option in getting-started.rst for better visibility.

^{This description was created by for 48dd40ced2d7e0b34d44016e5f999597ef8860d7. It will automatically update as commits are pushed.}

[Erudition]

Yeah, I just got burned as described - let's merge this please