Rewrite tasks documentation for new API

Completely rewrote the experimental tasks documentation to cover the new
simplified API and advanced features:

tasks.md (Overview):
- Clear task lifecycle diagram
- Bidirectional flow explanation (client↔server)
- Key concepts (metadata, store, capabilities)
- Quick example with new enable_tasks() + run_task() API

tasks-server.md (Server Guide):
- Quick start with enable_tasks() + run_task()
- Tool declaration (TASK_REQUIRED/OPTIONAL/FORBIDDEN)
- Status updates and progress
- Elicitation within tasks (form and URL modes)
- Sampling within tasks
- Cancellation support
- Custom task stores
- HTTP transport example
- Testing patterns
- Best practices

tasks-client.md (Client Guide):
- Quick start with poll_task() iterator
- Handling input_required status
- Elicitation and sampling callbacks
- Client as task receiver (advanced)
- Client-side task handlers
- Error handling patterns
- Complete working examples

Author: maxisbey