docs(config): add yaml-example doc-generator output and document V2 config blocks

[simonswine]

Summary

Picks up the work from @alliasgher in #5106:

• Adds a -format=yaml-example mode to the doc-generator and wires it into the Makefile so cmd/pyroscope/pyroscope.yaml is regenerated automatically.
• Registers the V2 root blocks (segment_writer, metastore, compaction_worker, query_backend, adaptive_placement, symbolizer, overrides_exporter) and removes the doc:"hidden" tags hiding them, so they appear in both the generated reference docs and the example config.

Credit: @alliasgher (original PR: #5106).

Test plan

• make generate produces no diff on a clean tree
• cmd/pyroscope/pyroscope.yaml reflects the V2 blocks
• docs/sources/configure-server/reference-configuration-parameters/index.md renders the new blocks

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Note

Low Risk
Low risk: changes are confined to documentation generation and example config output, with no impact to runtime behavior besides exposing previously hidden config fields in generated docs.

Overview
Adds a new -format yaml-example mode to tools/doc-generator (with -skip-blocks) to emit a commented YAML config example using default values and only basic fields.

Wires this into make generate to auto-regenerate cmd/pyroscope/pyroscope.yaml, and updates the generated reference docs to include previously hidden V2 top-level blocks (metastore, segment_writer, compaction_worker, query_backend, adaptive_placement, symbolizer, overrides_exporter) plus category annotations like (advanced)/(experimental).

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[alsoba13]

LGTM