Add a wiki page for guidelines on writing a validation job

[ankita240796]

Add a wiki page using https://docs.google.com/document/d/1u45oC6igsaTvQl4oNd8VvDiZe3JqeY3m_5n4QZ6d4rA/edit?usp=sharing to add pointers on creating a validation job.

This is an action item from https://github.com/oppia/oppia/pull/22225/files#r2115760390 and I plan to complete this by end of June'25.