Create documentation for the GCP storage source that includes the following information:

1. Introduction
   This section should describe the purpose of the extension and it's functionality within WiredTiger.
2. Build
   This section should describe how to build WiredTiger with the extension enabled. Be specific with version requirements for both the platform and any installed software.
3. Development
   This section should describe what an engineer needs to set up and install to develop locally. The instructions should work for all platforms supported by the extension (e.g. Linux, Mac, Windows). Be specific with version requirements for both the platform and any installed software.
4. Unit Testing
   This section should describe how to run the GCP unit tests from a developer machine and how a developer can add additional tests for the extension.
5. Evergreen Testing
   This section should describe the tasks defined in evergreen.yml for testing the extension code (that a developer would include in patch builds) and how a developer can add additional evergreen tests to the extension.

The documentation should be written as a README.md file in the  wiredtiger/ext/storage_sources/gcp_store directory.

Before submitting a PR for review, ask at least one WiredTiger engineer (who is not working on the project) to work through the instructions in your documentation.