Make ddsql preview private and ddsql default the index page

[estherk15]

What does this PR do? What is the motivation?

• Make the DDSQL Preview page private, and DDSQL Default page the new _index
• DOCS-11300

Merge instructions

Merge readiness:

• Ready for merge

For Datadog employees:

Your branch name MUST follow the <name>/<description> convention and include the forward slash (/). Without this format, your pull request will not pass CI, the GitLab pipeline will not run, and you won't get a branch preview. Getting a branch preview makes it easier for us to check any issues with your PR, such as broken links.

If your branch doesn't follow this format, rename it or create a new branch and PR.

[6/5/2025] Merge queue has been disabled on the documentation repo. If you have write access to the repo, the PR has been reviewed by a Documentation team member, and all of the required checks have passed, you can use the Squash and Merge button to merge the PR. If you don't have write access, or you need help, reach out in the #documentation channel in Slack.

Additional notes

[github-actions]

✅ Documentation Team Review

The documentation team has approved this pull request. Thank you for your contribution!

[github-actions]

Preview links (active after the build_preview check completes)

Removed or renamed files (these should redirect)

• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_default

Modified Files

• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/
• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_preview
• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_preview/data_types
• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_preview/ddsql_use_cases
• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_preview/expressions_and_operators
• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_preview/functions
• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_preview/reference_tables
• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_preview/statements
• https://docs-staging.datadoghq.com/esther/docs-11300-ddsql-reference-updates/ddsql_reference/ddsql_preview/tags

[maycmlee]

LGTM!

[timsara331]

LGTM 👍