MySQL: update MySQL connection instructions on how to make a readonly user

[Blargian]

Summary

Regarding https://clickhouse-inc.slack.com/archives/C03A9B0H05T/p1755032139630649, this PR updates the instructions for MySQL connection to recommend creating a readonly user with how to do so. Also improves the formatting of this snippet.

Checklist

• Delete items not relevant to your PR
• URL changes should add a redirect to the old URL via https://github.com/ClickHouse/clickhouse-docs/blob/main/docusaurus.config.js
• If adding a new integration page, also add an entry to the integrations list here: https://github.com/ClickHouse/clickhouse-docs/blob/main/docs/integrations/index.mdx

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Project	Deployment	Preview	Updated (UTC)
clickhouse-docs	✅ Ready	Preview	Aug 13, 2025 8:57am

[Blargian]

@mshustov could I please ask for your review on this one? As I understood it, we should use the readonly setting, in addition to granting only SELECT to the needed databases and tables?

[mshustov]

I'm not sure whether we need readonly = 1 (strict mode) or readonly = 2 (allow changing some settings https://clickhouse.com/docs/operations/settings/permissions-for-queries#readonly)