chore(block-theme): add readme.md to the search overlay block

[laurelfulford]

All Submissions:

• Have you followed the Newspack Contributing guidelines?
• Does your code follow the WordPress coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Matching what we've done for other block-theme related blocks so far, this PR adds a README.md for the Search Overlay block.

Closes NPPD-1539

How to test the changes in this Pull Request:

1. Read through the README.md and make sure it makes sense. The main gotcha with this block is that Jetpack's Instant Search will "take over" for it if enabled.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully run tests with your changes locally?

[Copilot]

Pull request overview

Adds a README.md for the Overlay Search block, matching the documentation pattern used for other block-theme-only Newspack blocks.

Changes:

• New README documents the block name, settings, styles, editor/front-end behavior, and Jetpack Instant Search handoff.
• Notes block-theme-only availability and the panel relocation/scroll-lock/contrast runtime behavior.