With the upcoming deprecation of the anonymous and service role keys,
this PR updates the Auth guides to use the publishable key instead of
the soon-to-be-deprecated anonymous key.
It also standardizes the example strings to be:
`'https://your-project-id.supabase.co'` and `'sb_publishable_...'` for
consistency.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
* **Documentation**
* Standardized client initialization examples to use a consistent
publishable-key placeholder (`sb_publishable_...`) and full project URL
format.
* Replaced "anon key" wording with "publishable key" across auth and API
guides and examples.
* Minor formatting and import-order/whitespace improvements in code
samples for clarity and consistency.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->