Update documentation to include SEARCH_API_KEY

[tacman]

Description

All of the examples show searching using the master key, but best practice is to use a search key and only use the master key for managing API keys.

Only use the master key to manage API keys. Never use the master key to perform searches or other common operations.

https://www.meilisearch.com/docs/learn/security/basic_security

I just discovered that there's a default search key, so I'm modifying my code to use it.