Remove redishll tracking docs

What does this MR do and why?

Redis/RedisHLL tracking is deprecated (see #413009 (closed)). This MR adjusts the docs accordingly to comply with the new structure for internal analytics docs discussed in gitlab-org/analytics-section/analytics-instrumentation/internal#670 (comment 1565203744).

The individual changes:

1. Move service_ping/metrics_instrumentation.md to metrics/metrics_instrumentation.md
2. Remove content specific to redis/redis_hll metrics from metrics/metrics_instrumentation.md
3. Move service ping debugging content from service_ping/implement.md to service_ping/troubleshooting.md
4. Move aggregate metrics content from service_ping/implement.md to metrics/metrics_instrumentation.md
5. Remove metrics/metrics_instrumentation.md
6. Remove Redis/RedisHLL metrics specific content from other pages

MR acceptance checklist

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