[DOC] Update Tempo docs for Agent > Alloy

[knylander-grafana]

Review the Tempo documentation to use Alloy instead of Agent.

Reference: grafana/pyroscope#3272
Related: https://github.com/grafana/tempo-squad/issues/440

Tasks

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1. Review existing documentation to see which pages reference Agent, include the set up and any config pages.
2. Are example configs in static or flow (river)?
3. Update set up pages in Tempo docs
4. Update other pages
5. Update Alloy docs to include traces for Migration from Staic
6. Update Alloy docs to include traces for Migrate from Flow
7. Verify with the Alloy writer

[knylander-grafana]

Update Agent >Alloy for Helm chart doc: https://github.com/grafana/tempo-squad/issues/440