API keys: Improve explanation of goals and use-cases for API keys

[guimachiavelli]

After one of his interactions with Cloud prospects, @qdequele reported the user was a bit at a loss of how they could use the feature. It seems the documentation works well enough at teaching people how to use API keys, but not why.

Currently waiting for more information, but it could be as simple as adding a couple of extra steps (try to change a setting with the search key, now try to do it with the admin key) and a "what's next" section.

Another alternative would be to add a short guide about scoped API keys.

References

• Slack discussion (private link): https://meilisearch.slack.com/archives/C07CLB9J4BH/p1739269051981939