docs(get-funded): add status notes for OP Retro Funding pause and Base Batches cohort windows

[Lesnak1]

Summary

Fixes #1723.

Adds clear status notes to docs/get-started/get-funded.mdx clarifying that:

1. OP Retro Funding is currently paused while Optimism reevaluates the program's long-term structure.
2. Base Batches operates in scheduled cohorts, with applications for the most recent cohort closed.

Rationale

Prevents builders reading the get-funded documentation from planning around paused or closed funding pathways, ensuring they focus on currently active grant opportunities.

[cb-heimdall]

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