Move tasks to individual files

[NathanHB]

What this does:

• move tasks from default_tasks into their own files
• move extended tasks into their own files
• move multilingual tasks into their own files
• move community tasks into their own files
• add docstring for default tasks
• add docstring for multilingual tasks
• add docstring for community tasks
• make a space to explore all the tasks, based on the tasks docstrings.
• modify the doc to reflect changes on how and where to add tasks

This allows for:

• better maintenance
• more readability (one task one file)
• have a nice task finder :)
• remove suites entirely

Also, getting rid of many duplicate tasks, for example, the same task in 2 different suites. I'm only keeping/moving all tasks to the lighteval suite so that we can eventually get rid of the suite system.

Viewer:

[HuggingFaceDocBuilderDev]

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