Serialise dates to strings for table of contents generator JSON dump

[Blargian]

Summary

If you try to generate a table of contents file containing dates, the script throws error:

TypeError: Object of type date is not JSON serializable

Adds default=str to json.dumps method such that dates will be written as strings. For our purposes this is probably sufficient for now.

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
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