Update current exit documentation with Capella fork epoch

[boulder225]

🎯 Problem to be Solved

The current exit documentation lacks specific information about using the "Capella fork epoch," which is 256 in the case of Holesky. This omission leads to issues with invalid voluntary exits, causing increased error rates and support tickets.

🛠️ Proposed Solution

• Update the exit documentation section Obol Quickstart Exit Docs.
• Introduce a subsection addressing the "Capella Fork Epoch" and its role in the exit process.
• State that the exit epoch should match the last fork's epoch to avoid invalid voluntary exits.
• [ ]Explain why aligning the exit epoch with the last fork's epoch is crucial. Provide examples for clarity.