docs: mark serverless as deprecated

[electron0zero]

What this PR does: we are deprecating serverless, so mark it as deprecated in the tempo docs

Which issue(s) this PR fixes: we are discussing it in https://github.com/grafana/tempo/issues/4014

Checklist

• [ ] Tests updated
• [x] Documentation added
• [ ] CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[joe-elliott]

I believe we are still discussing this: https://github.com/grafana/tempo/issues/4014

and would like to allow people from the community to give feedback before we mark this deprecated

[electron0zero]

will leave it as draft for time being, and give time for comments and feedback.

[github-actions[bot]]

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