Add task docs and default keybindings (#8123)

Also group task source modules together

Release Notes:

- N/A

---------

Co-authored-by: Piotr Osiewicz <24362066+osiewicz@users.noreply.github.com>

docs/src/tasks.md

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+# Tasks
+
+Zed supports ways to spawn (and rerun) commands using its integrated terminal to output the results.
+
+Currently, two kinds of tasks are supported, but more will be added in the future.
+
+## Static tasks
+
+Tasks, defined in a config file (`tasks.json` in the Zed config directory) that do not depend on the current editor or its content.
+
+Config file can be opened with `zed::OpenTasks` action ("zed: open tasks" in the command palette), it will have a configuration example with all options commented.
+
+Every task from that file can be spawned via the task modal, that is opened with `task::Spawn` action ("tasks: spawn" in the command pane).
+
+Last task spawned via that modal can be rerun with `task::Rerun` ("tasks: rerun" in the command palette) command.
+
+## Oneshot tasks
+
+Same task modal opened via `task::Spawn` supports arbitrary bash-like command execution: type a command inside the modal, and use `cmd-enter` to spawn it.
+
+Task modal will persist list of those command for current Zed session, `task::Rerun` will also rerun such tasks if they were the last ones spawned.